I was able to open and successfully repair a regular Nintendo GameCube controller (not a wavebird, a wired unit).

Yes, the triangular screws were interesting. I was able to use (if I recall correctly) a flathead jeweler's screwdriver to unscrew the little bastards. It was just a question of being really careful with it and using the right size.

In my case, the problem was just a dirty potentiometer on one of the analog axes. Your problem might be more complicated, don't know.