Making your fake guitar automatically whammy may be a step too far, but if you want to make Guitar Hero slightly easier for yourself, check out these guitar pedals. They allow you to either activate star power or hit the whammy bar with your feet instead of your hands, making sure your hands stay exactly where they're supposed to be to not miss a note.
Broken Boss Metal Zone pedal from pawn shop - $6
1/4" jack + patch cord - $5
Rip out the pedal's innards, solder the wires so the circuit closes when you press the pedal, put the jack into your guitar controller, soldering those contacts to the Select button. You're done, and it looks an awful lot cooler.
No whammy pedal, I guess, but whatever.