Sorry to bump an old thread but I thought I should share it here rather than make a new thread.
I recently bought a 4 in 1 adapter for the GC/Wii and after hearing a few horror stories I have to say it works perfectly for all the Emu's I've tried.
The adapter converts N64/NES/SNES/GEN-SMS. to GC controller (you could probably connect C64 Joysticks and others that use the same connector too but I haven't tried yet)
I was a bit apprehensive in buying one because when playing VC n64 games the way the adapter is made it swaps the R and Z trigger for some reason (probably an oversight on the makers part) but it works fine on Wii64.
So yeah, It works fine on Wii64 and it's probably cheaper than just the N64 standalone adapter.