Dude that's an awesome idea! Definitely going to try that when I bust it out again. Also, I see you're using transistors and pull up/down resistors. I recommend this Quad Bilateral Switch IC! It has 4 digital transistors on board. Power it with the Arduino, connect to ground, and all you have to do is set a pin High to enable the button press, and it condenses the circuit very well due to no resistors or individual transistors! I started with transistors like you but this was so much easier to manage. Use 4 of those ICs (~50 cents each) and you can control all the buttons.
I already documented my + & - on my controller, so I'm looking forward to trying this. I've never scratched off this black coating, how long does it take? I'm not sure what I'll be seeing under it.