http://kyorune.com/modding/article.php?id=80
same principle I think. I've never taken apart a GC so I'm not sure what the controller ports are like. Otherwise you could have a physical switch like this
That pic is for N64 but it's the same idea. Red wire is your power line for the controller. I would suggest hooking up ground to the switch too. I 've had some problems with just power.