Linux goesZen

For video and audio encoding in mencoder, you can tell mencoder to act as the front-end while the real work is done by the libav library in the backend. You do this by stating lavc as the codec for audio or video and then by passing additional lavcopts to the underlying avconv/ffmpeg library. Well, some switches you can pass are explained in the mencoder manual, while for others it simply says "look into the sources". For example, who would know that the avconv switch -qscale is vqscale in mencoder lingo..?!
For everyone too lazy to do that, here are the relevant parts of the current (2013-09) mencoder sources:

From libmpcodecs/ve_lavc.c (also look there for the default values, defined as constants)

const m_option_t lavcopts_conf[]={
{"acodec", &lavc_param_acodec, CONF_TYPE_STRING, 0, 0, 0, NULL},
{"abitrate", &lavc_param_abitrate, CONF_TYPE_INT, CONF_RANGE, 1, 1000000, NULL},
{"atag", &lavc_param_atag, CONF_TYPE_INT, CONF_RANGE, 0, 0xffff, NULL},
{"vcodec", &lavc_param_vcodec, CONF_TYPE_STRING, 0, 0, 0, NULL},
{"vbitrate", &lavc_param_vbitrate, CONF_TYPE_INT, CONF_RANGE, 4, MAX_BITRATE, NULL},
{"vratetol", &lavc_param_vrate_tolerance, CONF_TYPE_INT, CONF_RANGE, 4, MAX_BITRATE, NULL},
{"vhq", &lavc_param_mb_decision, CONF_TYPE_FLAG, 0, 0, 1, NULL},
{"mbd", &lavc_param_mb_decision, CONF_TYPE_INT, CONF_RANGE, 0, 9, NULL},
{"v4mv", &lavc_param_v4mv, CONF_TYPE_FLAG, 0, 0, 1, NULL},
{"vme", &lavc_param_vme, CONF_TYPE_INT, CONF_RANGE, 0, 8, NULL},
{"vqscale", &lavc_param_vqscale, CONF_TYPE_FLOAT, CONF_RANGE, 0.0, 255.0, NULL},
{"vqmin", &lavc_param_vqmin, CONF_TYPE_INT, CONF_RANGE, 1, 31, NULL},
{"vqmax", &lavc_param_vqmax, CONF_TYPE_INT, CONF_RANGE, 1, 31, NULL},
{"lmin", &lavc_param_lmin, CONF_TYPE_FLOAT, CONF_RANGE, 0.01, 255.0, NULL},
{"lmax", &lavc_param_lmax, CONF_TYPE_FLOAT, CONF_RANGE, 0.01, 255.0, NULL},
{"mblmin", &lavc_param_mb_lmin, CONF_TYPE_FLOAT, CONF_RANGE, 0.01, 255.0, NULL},
{"mblmax", &lavc_param_mb_lmax, CONF_TYPE_FLOAT, CONF_RANGE, 0.01, 255.0, NULL},
{"vqdiff", &lavc_param_vqdiff, CONF_TYPE_INT, CONF_RANGE, 1, 31, NULL},
{"vqcomp", &lavc_param_vqcompress, CONF_TYPE_FLOAT, CONF_RANGE, 0.0, 1.0, NULL},
{"vqblur", &lavc_param_vqblur, CONF_TYPE_FLOAT, CONF_RANGE, 0.0, 1.0, NULL},
{"vb_qfactor", &lavc_param_vb_qfactor, CONF_TYPE_FLOAT, CONF_RANGE, -31.0, 31.0, NULL},
{"vmax_b_frames", &lavc_param_vmax_b_frames, CONF_TYPE_INT, CONF_RANGE, 0, FF_MAX_B_FRAMES, NULL},
{"vpass", &lavc_param_vpass, CONF_TYPE_INT, CONF_RANGE, 0, 3, NULL},
{"vrc_strategy", &lavc_param_vrc_strategy, CONF_TYPE_INT, CONF_RANGE, 0, 2, NULL},
{"vb_strategy", &lavc_param_vb_strategy, CONF_TYPE_INT, CONF_RANGE, 0, 10, NULL},
{"vb_qoffset", &lavc_param_vb_qoffset, CONF_TYPE_FLOAT, CONF_RANGE, 0.0, 31.0, NULL},
{"vlelim", "Please use o=luma_elim_threshold=<value> instead of vlelim.\n", CONF_TYPE_PRINT, 0, 0, 0, NULL},
{"vcelim", "Please use o=chroma_elim_threshold=<value> instead of vcelim.\n", CONF_TYPE_PRINT, 0, 0, 0, NULL},
{"vpsize", &lavc_param_packet_size, CONF_TYPE_INT, CONF_RANGE, 0, 100000000, NULL},
{"vstrict", &lavc_param_strict, CONF_TYPE_INT, CONF_RANGE, -99, 99, NULL},
{"vdpart", &lavc_param_data_partitioning, CONF_TYPE_FLAG, 0, 0, 1, NULL},
{"keyint", &lavc_param_keyint, CONF_TYPE_INT, 0, 0, 0, NULL},
{"gray", &lavc_param_gray, CONF_TYPE_FLAG, 0, 0, CODEC_FLAG_GRAY, NULL},
{"mpeg_quant", &lavc_param_mpeg_quant, CONF_TYPE_FLAG, 0, 0, 1, NULL},
{"vi_qfactor", &lavc_param_vi_qfactor, CONF_TYPE_FLOAT, CONF_RANGE, -31.0, 31.0, NULL},
{"vi_qoffset", &lavc_param_vi_qoffset, CONF_TYPE_FLOAT, CONF_RANGE, 0.0, 31.0, NULL},
{"vqsquish", &lavc_param_rc_qsquish, CONF_TYPE_FLOAT, CONF_RANGE, 0.0, 99.0, NULL},
{"vqmod_amp", &lavc_param_rc_qmod_amp, CONF_TYPE_FLOAT, CONF_RANGE, 0.0, 99.0, NULL},
{"vqmod_freq", &lavc_param_rc_qmod_freq, CONF_TYPE_INT, 0, 0, 0, NULL},
{"vrc_eq", &lavc_param_rc_eq, CONF_TYPE_STRING, 0, 0, 0, NULL},
{"vrc_override", &lavc_param_rc_override_string, CONF_TYPE_STRING, 0, 0, 0, NULL},
{"vrc_maxrate", &lavc_param_rc_max_rate, CONF_TYPE_INT, CONF_RANGE, 0, MAX_BITRATE, NULL},
{"vrc_minrate", &lavc_param_rc_min_rate, CONF_TYPE_INT, CONF_RANGE, 0, MAX_BITRATE, NULL},
{"vrc_buf_size", &lavc_param_rc_buffer_size, CONF_TYPE_INT, CONF_RANGE, 4, MAX_BITRATE, NULL},
{"vrc_buf_aggressivity", &lavc_param_rc_buffer_aggressivity, CONF_TYPE_FLOAT, CONF_RANGE, 0.0, 99.0, NULL},
{"vrc_init_cplx", &lavc_param_rc_initial_cplx, CONF_TYPE_FLOAT, CONF_RANGE, 0.0, 9999999.0, NULL},
{"vrc_init_occupancy", &lavc_param_rc_initial_buffer_occupancy, CONF_TYPE_FLOAT, CONF_RANGE, 0.0, 1.0, NULL},
{"vfdct", &lavc_param_fdct, CONF_TYPE_INT, CONF_RANGE, 0, 10, NULL},
{"aspect", &lavc_param_aspect, CONF_TYPE_STRING, 0, 0, 0, NULL},
{"autoaspect", &lavc_param_autoaspect, CONF_TYPE_FLAG, 0, 0, 1, NULL},
{"lumi_mask", &lavc_param_lumi_masking, CONF_TYPE_FLOAT, CONF_RANGE, -1.0, 1.0, NULL},
{"tcplx_mask", &lavc_param_temporal_cplx_masking, CONF_TYPE_FLOAT, CONF_RANGE, -1.0, 1.0, NULL},
{"scplx_mask", &lavc_param_spatial_cplx_masking, CONF_TYPE_FLOAT, CONF_RANGE, -1.0, 1.0, NULL},
{"p_mask", &lavc_param_p_masking, CONF_TYPE_FLOAT, CONF_RANGE, -1.0, 1.0, NULL},
{"naq", &lavc_param_normalize_aqp, CONF_TYPE_FLAG, 0, 0, 1, NULL},
{"dark_mask", &lavc_param_dark_masking, CONF_TYPE_FLOAT, CONF_RANGE, -1.0, 1.0, NULL},
{"ildct", &lavc_param_interlaced_dct, CONF_TYPE_FLAG, 0, 0, 1, NULL},
{"idct", &lavc_param_idct, CONF_TYPE_INT, CONF_RANGE, 0, 20, NULL},
{"pred", &lavc_param_prediction_method, CONF_TYPE_INT, CONF_RANGE, 0, 20, NULL},
{"format", &lavc_param_format, CONF_TYPE_IMGFMT, 0, 0, 0, NULL},
{"debug", &lavc_param_debug, CONF_TYPE_INT, CONF_RANGE, 0, 100000000, NULL},
{"psnr", &lavc_param_psnr, CONF_TYPE_FLAG, 0, 0, CODEC_FLAG_PSNR, NULL},
{"precmp", &lavc_param_me_pre_cmp, CONF_TYPE_INT, CONF_RANGE, 0, 2000, NULL},
{"cmp", &lavc_param_me_cmp, CONF_TYPE_INT, CONF_RANGE, 0, 2000, NULL},
{"subcmp", &lavc_param_me_sub_cmp, CONF_TYPE_INT, CONF_RANGE, 0, 2000, NULL},
{"mbcmp", &lavc_param_mb_cmp, CONF_TYPE_INT, CONF_RANGE, 0, 2000, NULL},
{"skipcmp", &lavc_param_skip_cmp, CONF_TYPE_INT, CONF_RANGE, 0, 2000, NULL},
#ifdef FF_CMP_VSAD
{"ildctcmp", &lavc_param_ildct_cmp, CONF_TYPE_INT, CONF_RANGE, 0, 2000, NULL},
#endif
{"bit_exact", &lavc_param_bit_exact, CONF_TYPE_FLAG, 0, 0, CODEC_FLAG_BITEXACT, NULL},
{"predia", &lavc_param_pre_dia_size, CONF_TYPE_INT, CONF_RANGE, -2000, 2000, NULL},
{"dia", &lavc_param_dia_size, CONF_TYPE_INT, CONF_RANGE, -2000, 2000, NULL},
{"qpel", &lavc_param_qpel, CONF_TYPE_FLAG, 0, 0, CODEC_FLAG_QPEL, NULL},
{"trell", &lavc_param_trell, CONF_TYPE_FLAG, 0, 0, 1, NULL},
{"lowdelay", &lavc_param_lowdelay, CONF_TYPE_FLAG, 0, 0, CODEC_FLAG_LOW_DELAY, NULL},
{"last_pred", &lavc_param_last_pred, CONF_TYPE_INT, CONF_RANGE, 0, 2000, NULL},
{"preme", &lavc_param_pre_me, CONF_TYPE_INT, CONF_RANGE, 0, 2000, NULL},
{"subq", &lavc_param_me_subpel_quality, CONF_TYPE_INT, CONF_RANGE, 0, 8, NULL},
{"me_range", &lavc_param_me_range, CONF_TYPE_INT, CONF_RANGE, 0, 16000, NULL},
{"aic", &lavc_param_aic, CONF_TYPE_FLAG, 0, 0, CODEC_FLAG_AC_PRED, NULL},
{"umv", &lavc_param_umv, CONF_TYPE_FLAG, 0, 0, 1, NULL},
{"aiv", &lavc_param_aiv, CONF_TYPE_FLAG, 0, 0, 1, NULL},
{"obmc", &lavc_param_obmc, CONF_TYPE_FLAG, 0, 0, 1, NULL},
{"loop", &lavc_param_loop, CONF_TYPE_FLAG, 0, 0, CODEC_FLAG_LOOP_FILTER, NULL},
{"ibias", &lavc_param_ibias, CONF_TYPE_INT, CONF_RANGE, -512, 512, NULL},
{"pbias", &lavc_param_pbias, CONF_TYPE_INT, CONF_RANGE, -512, 512, NULL},
{"coder", &lavc_param_coder, CONF_TYPE_INT, CONF_RANGE, 0, 10, NULL},
{"context", &lavc_param_context, CONF_TYPE_INT, CONF_RANGE, 0, 10, NULL},
{"intra_matrix", &lavc_param_intra_matrix, CONF_TYPE_STRING, 0, 0, 0, NULL},
{"inter_matrix", &lavc_param_inter_matrix, CONF_TYPE_STRING, 0, 0, 0, NULL},
{"cbp", "Please use o=mpv_flags=+cbp_rd instead of cbp.\n", CONF_TYPE_PRINT, 0, 0, 0, NULL},
{"mv0", &lavc_param_mv0, CONF_TYPE_FLAG, 0, 0, CODEC_FLAG_MV0, NULL},
{"nr", &lavc_param_noise_reduction, CONF_TYPE_INT, CONF_RANGE, 0, 1000000, NULL},
{"qprd", "Please use o=mpv_flags=+qp_rd instead of qprd.\n", CONF_TYPE_PRINT, 0, 0, 0, NULL},
{"ss", &lavc_param_ss, CONF_TYPE_FLAG, 0, 0, 1, NULL},
{"alt", &lavc_param_alt, CONF_TYPE_FLAG, 0, 0, 1, NULL},
{"ilme", &lavc_param_ilme, CONF_TYPE_FLAG, 0, 0, CODEC_FLAG_INTERLACED_ME, NULL},
{"cgop", &lavc_param_closed_gop, CONF_TYPE_FLAG, 0, 0, CODEC_FLAG_CLOSED_GOP, NULL},
{"gmc", &lavc_param_gmc, CONF_TYPE_FLAG, 0, 0, CODEC_FLAG_GMC, NULL},
{"dc", &lavc_param_dc_precision, CONF_TYPE_INT, CONF_RANGE, 8, 11, NULL},
{"border_mask", &lavc_param_border_masking, CONF_TYPE_FLOAT, CONF_RANGE, 0.0, 1.0, NULL},
{"inter_threshold", "inter_threshold has no effect, please remove it.\n", CONF_TYPE_PRINT, 0, 0, 0, NULL},
{"sc_threshold", &lavc_param_sc_threshold, CONF_TYPE_INT, CONF_RANGE, -1000000000, 1000000000, NULL},
{"top", &lavc_param_top, CONF_TYPE_INT, CONF_RANGE, -1, 1, NULL},
{"qns", "Please use o=quantizer_noise_shaping=<value> instead of qns.\n", CONF_TYPE_PRINT, 0, 0, 0, NULL},
{"nssew", &lavc_param_nssew, CONF_TYPE_INT, CONF_RANGE, 0, 1000000, NULL},
{"threads", &lavc_param_threads, CONF_TYPE_INT, CONF_RANGE, 1, 16, NULL},
{"turbo", &lavc_param_turbo, CONF_TYPE_FLAG, 0, 0, 1, NULL},
{"skip_threshold", &lavc_param_skip_threshold, CONF_TYPE_INT, CONF_RANGE, 0, 1000000, NULL},
{"skip_factor", &lavc_param_skip_factor, CONF_TYPE_INT, CONF_RANGE, 0, 1000000, NULL},
{"skip_exp", &lavc_param_skip_exp, CONF_TYPE_INT, CONF_RANGE, 0, 1000000, NULL},
{"brd_scale", &lavc_param_brd_scale, CONF_TYPE_INT, CONF_RANGE, 0, 10, NULL},
{"bidir_refine", &lavc_param_bidir_refine, CONF_TYPE_INT, CONF_RANGE, 0, 4, NULL},
{"sc_factor", &lavc_param_sc_factor, CONF_TYPE_INT, CONF_RANGE, 1, INT_MAX, NULL},
{"vglobal", &lavc_param_video_global_header, CONF_TYPE_INT, CONF_RANGE, 0, INT_MAX, NULL},
{"aglobal", &lavc_param_audio_global_header, CONF_TYPE_INT, CONF_RANGE, 0, INT_MAX, NULL},
{"mv0_threshold", &lavc_param_mv0_threshold, CONF_TYPE_INT, CONF_RANGE, 0, INT_MAX, NULL},
{"refs", &lavc_param_refs, CONF_TYPE_INT, CONF_RANGE, 1, 16, NULL},
{"b_sensitivity", &lavc_param_b_sensitivity, CONF_TYPE_INT, CONF_RANGE, 1, INT_MAX, NULL},
{"level", &lavc_param_level, CONF_TYPE_INT, CONF_RANGE, INT_MIN, INT_MAX, NULL},
{"o", &lavc_param_avopt, CONF_TYPE_STRING, 0, 0, 0, NULL},
{"audio_o", &lavc_param_audio_avopt, CONF_TYPE_STRING, 0, 0, 0, NULL},
{NULL, NULL, 0, 0, 0, 0, NULL}
};