It's pretty easy to change the redirect itself. The relevant key is HKCR\vnd.youtube, which defines a URI handler (the way the redirect works is that when YouTube's servers see the WP7 browser user-agent, they send links like "vnd.youtube:<video ID>" which WP7 then opens in the YouTube app). Changing two registry values under that key, replacing the App ID of the YouTube app with the ID of the HTC app, would make those links open in the HTC app.
The problem is that the HTC app doesn't expect to be opened this way, so it won't check for command line parameters. Instead, it'll just open as though you'd selected it from the app list. This probably isn't the behavior you're hoping for.