if it is possible to add digital output could it be possible to add a digital input?

In theory yes. But in practise, no. The lines that go to the tuner can be reconfigured to operate as a digital input but even the designers of the empeg never actually tried it.

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.

There already is a possible method for getting the sound data in and it is connected to the correct DSP input as well.

Not according to what I can see on the circuit diagrams, there isn't.

That is what Hugo said.

Didn't say it's for audio - just pointing that it's there: there's also three I/O lines on the ethernet, plus a few other unused bits.

No, there are three I/O lines on the chip brought out to pads on the PCB. I am not sure if the driver supports the use of the lines though (ioctrl?).

Funny you should mention that.....