I guess I'll add my design ideas.
Accorss the top of my cp I have the following buttons.
Pause, Advance, Coin1, P1 Start, P2 Start, Coin2, Tilde(~), Tab, Enter, Save, Pause(Wired in paralell with the other pause button).
Tilde, Tab and Enter are all wired to a hidden switch to enable them.
Coin1 is the shift button. The Shifted functions are:
Esc,Reset,Shift,P1 Start,P2 Start,Coin2,Tilde(~),Tab,Enter,Load,Esc.
All of my The player buttons and joystick inputs are setup so that it does not matter if the shift buttin is pushed.