Yeah, I guess my question is how to get the configuration info for the buttons to the arduino from within PiPlay. E.g. If I open Asteroids, it lights up only those buttons -- how do I "push" data from the Pi to the Arduino?
Blacksheep32 -- is that the route you're going, or what are you thinking?
I am working on a solution to use I2C for the controllers and then use the remaining IO to control solid state relays to turn on associated lights right off the Pi. You could use the arduino as an I2C device and do the same. So that the arduino is slaved to the Pi and depending on which emulator is selected (True) a specific light combination is lit up.
That's what I was thinking as well. Light them up on a per emulator basis, but I think that @mkcroce is hoping to light them on a per game basis, in which case you would need to make a list of the games along with what buttons that they need to light (as far as I can tell, there's no way to get that info from the emulator)