I don't think it can be done. Does an iPod connect to the USB port, or does it use an iPod cable? I would be surprised if it controlled the iPod through USB, they usually use a separate iPod cable with a line-out and a serial port.
The USB port is there for a Mass-Storage device only, so the best the computer could do is emulate a USB-flash device with the contents of the HDD (or a specific folder) on it. Even that would be very hard.
Now if it really did control the iPod and play its audio all though a standard USB-to-iPod cable, then it is technically possible, but making a computers USB-Host emulate an iPod-USB-Device would also be very very hard.
On the other hand, making a cable that connects an iPod cable to your analog audio-out and a serial port is much easier. Mitch JS sells a cable for controlling an iPod using a PC. If you just had a female-to-female iPod-connector you could use his cable, but you would still need to write all the software to emulate an iPod's serial port (rather then control an iPod via the serial port).