USA version works on my USA Wii.

Try patching the ISO? I don't know what the utility is called, but you can region patch any gamecube iso. You can also boot to it through GCOS, I think the WiiKey setup disk has this utility in it.

The region free features of the WiiKey are only for Wii games, since the GC games are region coded differently.

 

Nah, the Wiikey region freeness works for Gamecube games burned (single) to a DVD. With my USA Wii, I cannot play PAL Gamecube games through Make-Multi's setup, but only when I burn them directly to a DVD (+RW to be exact).