I use more buttons than randoms (for example, I use the '_Z' and that one doesn't enter in the random of 2-3 I have) and all the buttons work.
One has to hold to be sure. I use combinations like A+R, or simply A. As I use A for selecting the stage, if I'm holding R already, my combo always gets detected.
No Gamecube ports because newer Wii.
It also seems to not work on Dolphin, always loads _A stage.