Someone was trying to convince me to support wallboxes at one time. They use a simple voltage pulse to send the selection to the main juke, which would theoretically be readable through a simple parallel port interface. I couldn't see enough benefit to go through the effort of getting my hands on a working wallbox, working out the code, designing a parallel port interface, and writing the code. And that would only be able to support up to 8 boxes without getting into some fancy multiplexing scheme.
Maybe someday... if I'm going to support external hardware, though, I'd be more likely to work on something for all these new lighting controllers or an 4x40 LCD display or an IR remote control.