It's actually easier to change the directory for ps3 games, just rename it Gamez and use games for your Wii/GC stuff. I too have a couple ps3 games on my wii hdd in this fashion, they get picked up automatically by MultiMan without having to adjust any settings iirc
Alternatively, I'm not 100% sure but I think if you change the directory for gc games in usbloader GX's settings, it will load nintendont using the alternate path instead of the default games directory. There may be another way to do this more directly using Nintendont, but I'm not sure.