Oh I didn’t set out to address some sort of limitation with the subtitles filter, and I’m sure everyone else is doing fine with it (on Linux?). But, I’m afraid I can’t give you a good or adequate/qualified answer, except for: I can’t get it to work (I’m embarrassed actually).
I remember seeing video conversion apps with ffmpeg as main back-end, but relegating subtitle-burning-in tasks to ?mencoder? which I do not like.
Anyway, I’m on Windows XP using ffmpeg-20140226-git-96fc290-win32-static. I seem to recall seeing on the log file: ‘neither playresx nor playresy defined’, reverting to 384 x some other number, a bunch of subtitle-related stuff which seemed to indicate that it can manage and everything is okay, but no subs for me. I’ve given up on this a long time ago (sorry I couldn’t give a detailed answer). I’m not complaining, though, it just so happens that an alternative (sendcmd & drawtext) exists in ffmpeg. Just another way to skin a cat, I suppose (for those having trouble doing it the proper way).