Even if you can get MythTV to work via DirectFB/QT Embedded, i think that MythTV will try to use FFmpeg codecs(ffmpeg uses cpu, not the SMP DSP's) to decode the video... And the poor 300mhz cpu simply don't have power to decode video... So, unless the DirectFB version of SMP863x has some kind of abstraction to mrua libs for video decoding, you'll have to patch MythTV(or libavcodec) to use mrua...