If by hacking a gamecube, you mean to connect it to arcade controls, i think that it is probably doable. However, it is extremely difficult (or nearly impossible) to interface the analog joystick to arcade controls. This means you are limited to games that use the digital joypad, which for the gamecube is rather small.
Still you would be able to play some sweet games like ikaruga, soul caliber 2 and (maybe?)smash brothers. Check out this console hacking site. Although they only have instuctions for dreamcast and x-box controllers, the idea is the same for gamecube.
http://members.cox.net/joysticks/