The USB port in the empeg is only a slave port. You can't connect another slave device to it easily. In theory you could make a device which has USB master support and plug the empeg and another slave into it. The device would arbitrate the connection between the two but it's not been done before and also you'd need to write a kernel driver to support this.

In theory if you bought one of Patrick's new tuners and his yet to be released expansion module you could... But at the moment there isn't anything ready made that does it. I'd say the simplest method would be to program a PIC chip that you'd plug into the serial port on the sled. Still leaves the matter of the software but it shouldn't be that hard to do.

The IrDA port in the empeg isn't enabled by default. It's actually a serial port on the StrongARM chip with a IrDA transceiver connected to it. Kim disconnected the IrDA chip and wired in an Ericsson Bluetooth module instead. He put some photos of it here

There is an I2C bus inside the empeg already. I believe the DSP and a few other chips are already connected to it. Benjammin made a I2C fan module which connects to it. His schematic is here