Just bored reading at work, I came across a few bits of info that got me interested:

From Yet another Gamecube Documentation:
"SNSystems reports that their ProDG development kit can be used with a special USB adapter of theirs for directly sending program (debug) code to NR-Reader GameCubes."

From Wikipedia:
"the cable appears to be protruding from the left side of the case where the Serial Port 2 should be. If this does connect to that port, this would be the only accessory known to do this."

So, did the ProDG USB cable connect in through the otherwise unused serial port 2? And if so, I'd very much like to see a close up photo or two of it. If anyone has one, or has come across pictures in the past, please post them! :)