If you can program the serial touchpad to:

1. Work at either 4800 or 115200 BPS.

2. Work with no flow control.

3. Output a specific set of commands ...

Then one should just plug directly in.

If #3 there is not possible with the serial touchpad you want to use, then there's likely a way to write a piece of software or a kernel tweak for the empeg to respond to whatever it *does* output.