Well I'm sure it could be possable to make a USB/SD Loader for the GC on Wii because on the GameCube they made an Action Replay whitch also has an adapter type thing from memcard to SD card so if they made some software similare to the Action Replay and it loads from the Memery Card Slot B then it should be possable to play games and load your save data from the slot A on a SD card.
Also if they messed around with the action replay adapter they might be able to end it with a USB port instead of a SD card slot then it could load of a USB.
I'm not exactly sure if this would work but i think it would.