No you can't load GC games from USB. You're only option would be to buy a WODE or I believe a SunDriver will also allow you to play GC backups.
Can I load GameCube ISOs onto an external HDD and play via USB Loader GX just as I would with Wii ISOs? If not, is there a way that I can play GameCube backups? I have a softmodded 4.3U black Wii so it doesnt play burned discs.
Hmm, okay thanks for the info. I guess I'll just continue buying GameCube games, I have most of the ones I want anyway. I just wanted to play Soul Calibur II today, aha.