I've uploaded a new version of mythtv (0.21-maemo1), which supposedly has the cpu optimizations built in finally. Let me know if you notice any differences.

https://garage.maemo.org/frs/?group_id=632

The good thing is mplayer and mythtv share common code for the decoding part (both use libavcodec from ffmpeg) however the videoout code is different. So I may try copying in mplayer's libavcodec, and if that doesn't work I may just add a "mplayer" playback method.