I tested this yesterday with Mario 64 NTSC-U iND release and region patched it to region free. It worked fine on my PAL Wii, also the game runs at full screen 60hz which faster and smoother than than the optimized PAL Mario 64.

It wont work on all games though, i tried to region free Star Fox 64 NTSC-U. It installed fine but the game was still running at 50hz and sound was skipping.