You can set the maemo_allow_custom_player variable (in the advanced preferences) to True and then set the player (for audio) and videoplayer configuration variables to a command that will be invoked when you play an audio or video file.

Be aware that setting maemo_allow_custom_player to True disables the Nokia Mediaplayer integration - you have to specify the "player" command in addition to the "videoplayer" command to get audio playback working, too.

That said, setting "videoplayer" to "mplayer" should work, although the internet tablets might have a hard time playing back high-quality video content.. flv might work.