A cool idea, but I think it is unlikely.  Sounds like touchpad is running over existing remote control software.  You need some kind of listener running on the pc that can receive the mouse information from Chumby and interface with the mouse driver.   If that isn't hard enough, you have to get Chumby to send that information somehow, and from what I understand it is a pretty good trick just to convince Chumby to see files on local computers.  I suppose if the listener on your PC was a web server of sorts, then Chumby might have a chance of talking to it.