Have you looked at using an airpanel from Viewsonic?[Link: viewsonic.com]
It is basically a handheld PC that can run as a remote monitor. In essence, you would run windows on your HTPC and operate the HTPC remotely. Anything you could do sitting in front of your HTPC you could do with an airpanel.
I believe you could do the same with Viewsonic's viewpad as well....those can be had used on ebay for a few hundred $$$.