That all depends on the built-in unit. Most units don't give you easy access to the touch screen, though there are a few (the Prius is the only one I actually know of for certain though). Even if you can access the TS, it is doubtful that you can get it to act like a TS design for a "real" computer (e.g. not a task specific job like a Nav system).
You mention using the A/V input and I am guessing that you are referring to feeding the video in that was as well. If so, know that while it is possible most people are not happy with it at all. The problem is that an A/V screen doesn't have a good enough resolution to display your standard computer display. For FrontRow you might be happy enough with it, but web browsing will be slightly better than useless as normal text is where most of the problems are seen. Just something to consider if you haven't looked into that issue yet and i'm sure BugByte will be by shortly with his links to threads about this topic