There's one which is a LiveCD... you run it then when it's loaded in memory you eject the CD and stick your DVD in the drive and watch it in mplayer. Was compatible with DivX/XviD too.
Can't remember the name off the top of my head... might have to go rummage around my old discs to see if I can find it.
edit: found it, it was called MoviX. Not sure if it's suitable for what you're looking to do though, but might be worth a look.