hmm when I looked in this thread I was hoping to see some discussion involving bushing's newest post ("of TMD's and hardware") but I see none.
It may be possible with some effort to create a Gamecube usb loader and a gamecube sd loader. It might even be possible to revive old projects that loaded games from the broadband adapter over the wifi instead (less sure about this, could be more difficult).
Since we can now enable access to all the wii's hardware from ppc, we can have full access to the best of gamecube mode (direct access to dvd drive and memory) and wii mode (ability to use wifi, bluetooth, usb ports, sd card reader...). However, since the gamecube games do not make use of ios, I think it will be necessary to patch them to make the required ios calls, and run them under a cIOS (not cMIOS). The easiest project to port would be sdboot, since I'm guessing we can change its callback function to make requests to ios instead of directly accessing the memory card slots.
I think bushing suggests that all hardware can be re-enabled through the following commands, but I'm guessing these need to be executed from ios, so it would require a further cIOS revision, which I'm sure waninkoko will deliver soon.
#define HW_MEMMIRR 0x0d800060
#define HW_AHBPROT 0x0d800064
set32(HW_MEMMIRR, 8);
set32(HW_AHBPROT, 0xFFFFFFFF);
Perhaps the real question is, is it worth the effort at this point? Hopefully someone will work it out, just for the sake of completeness.