If you wired them to a resistor network, you could plug it into the tuner module and it would work like the steering wheel control interface. At that point, a kernel hack could remap the values to any IR commands the player would normally interpret.

If you wired them up to a custom PIC like in John Lambon's system, then you could program the PIC to send whatever ASCII codes you wanted to the player's serial port. You could either use existing serial port commands or come up with some kind of hack to re-interpret those commands.