To install a coin aceptor, take a look at http://forums.adafruit.com/viewtopic.php?f=8&t=44883
For now the frontend is tightly bound to the customized version of raspbian, and only accept gpio controls as inputs, in a few months will be released a modified version of the frontend, with gamepads or keyboard controls.
Use a background screen for each game: ops, this is posible, but its a lot of work make a background screen for each game. We will try to make something for the next release.
If I can help you, do not hesitate to ask.