There is a documentation error -- https://trac.ffmpeg.org/ticket/11678 has been submitted. dvdsubs can be muxed into mp4, and even 5 year-old players can play the mp4 and show the subtitles. I suspected this was true because mp4 is MPEG, but I'd read that the bit-mapped subs have to be burned in. That's not true. Invoke the dvd_subtitle codec via this (corrected) option: -c:s dvd_subtitle --Mark.