Well, the sound o/p to the digital board is I2S, so the same bus could be used to transfer sound data. Problem is, there's no receiving device on the bus that could do something with it, so even if you made an electrical connection that was I2S compliant, it would not be useful.

There is also an I2C bus, as has been demonstrated by Benjamin's fan controller. This requires a device driver in the kernel if you want to use it from application userland: however, you'd have to write the driver and build the kernel before even starting on the application to use it. The player app would do nothing with it - only (potentially) the HiJack kernel, if you talked Mark into adding support for something.