It was also used on the wii for various debugging purposes (
http://wiibrew.org/wiki/USB_Gecko and the SD adapter stuff ) and the like so that might be a good place to also look, certainly a lot of people wanting to pull things from and stick things on GC memory cards were told to get a wii and use that but that was probably more because it came with network and SD options.
Still for the GC itself I assume you have seen yagcd, various chapters covering everything from pinout to the protocol and ways devices seen in the wild worked in there.
http://hitmen.c02.at/files/yagcd/yagcd/frames.html
Don't know what the dolphin source code might have but in some regards it has taken over from yagcd, though in others it certainly has not. A quick scan says something but probably not a lot
https://github.com/dolphin-emu/dolphin/search?utf8=✓&q=memory+card&type=