x264 should now be able to encode compliant AVC-Intra 50.
With a 10-bit-compiled version of x264, a sample commandline for 1080i25 might be:
--interlaced --keyint 1 --vbv-bufsize 2000 --bitrate 50000 --vbv-maxrate 50000 --nal-hrd cbr

Also print "Constrained Baseline" for baseline profile, since that's all x264 (and everything else in the world) supports.
Also reorganize parameter validation a bit to reduce some spurious warnings.