An original xbox is a great media server or mame box but you cannot run the driver for the touchscreen on the xbox so it will probably not work even thou you can connect via modified usb.
Sorry mate but you are wrong.
If you have a touch screen film, that is compatible with a USB mouse (I have a trashed EEEPC that has one) it just plugs in and is detected by XBMC. I'm not sure about custom touch screens, but the one I linked works like a mouse.
I would suggest that you do some research before making such comments, as it is very unhelpful if someone is making a project on the cheap. The only problem I saw in getting to work was getting the screen configured for a 7" work area.
I connect to the XBMC via my android and WiFI. If I was going to do something like this I would skip the touchscreen affair and look at getting a cheap tablet. That way you can use the tablet for home use and then dock it to your cab and use the XBMC app.
There is a new android app out that runs an app on charge event, so when you dock the tablet it will go into XBMC automatically and use it as a juke function. I tried to make a pic of this but it is too dark. You can pick up that Apad for $99 so it would be a better option.