I just wanted to report that on the Pioneer 444 and on the Koda 2233 DVD player I've succeeded in mixing (or should I say muxing) MP3 audio with a 704x576 resolution mpeg 2 file.

Simply use TMPGEnc multiplexing functions and play.

I've so far only used a music mp3, not the actual movie's sound so I dont know if you get sound sync issues etc. I do know tmpgenc is picky about what mp3's it accepts at all. I suggest using CBR at least.