The THP video format used in many Wii games is well understood, but I don't think anyone has made an encoder for that format yet. There are also many variations of this format, which doesn't make anything easier.
Downsampling audio should not be necessary for most games, but I think that the 3x reading speed might be a problem only when playing back multi-brstm audio (Mario Kart Wii and Super Mario Galaxy). I believe there is an encoder for brstm audio, but there is probably no encoder that can produce multi-brstm files...