WBFS isn't used for wii discs. it is a system created by non nintendo developers..Wii Backup File system.
as far as a multiboot dvd with different types of games i don't think there is a way to do it yet. it would take new loader that would boot BC if a game partition was GC and boot the game as normal if it was wii. the reason it wont work with the corrent stuff is because the disc itself can only have 1 magic word in it. currently, all GC multiboot discs appear to the wii as a GC game and Wii games appear as a Wii disc.
also, with a wii multiboot, the loader is in the wii (or sd card). on a GC multiboot, the loader is on the disc itself.