Hey Plecto, you would need a microcontroller at least to interpret the input (touches) and change them into something your HTPC can understand. You would also need a microcontroller to run the display build menus etc. More interfacing to convert it all into a wireless signal, and some in your pc to receive it. If it has a composite input, then I suppose it might be possible to use a wireless video sender ($50?) from your htpc to send video to it, and then configure it as a secondary display. It would essentially be a whole display that would display your buttons, menus etc. You would still need to do the reverse to get the control signals back into your pc. Depends what kind of interface the touch sensors has built it - maybe it'd be possible to emulate a mouse, and there are plenty of wireless mice around. Probably depends on what interfaces are already built in to the screen.
Anything's possible with the right resources, but Bill's right, spend $150 on a cheapie android tablet an get an reusable, ideal solution, or spend $100 and do six months work to get something that only does one thing.