This has been asked hundreds of times. A GC USB Loader is extremely difficult if not impossible.
There is simply no USB support in gamecube mode. Adding USB support to MIOS would be very involved (if it didn't end up being rewritten completely because of how GC games access hardware). Then we'd have to convince the wii to not turn the ports off in GC mode.
There was something recently mentioned about developing a special MIOS with a DI module in it for SNEEK (i think they called it DIOS), but I'm not sure if it was real or not. I saw it a while back, likely around April 1...