On the projection screen you would use a IR Pen. Something like this -
The hardest part would probably be getting the right alignment and practicing the demo to make sure you did not block the beam. Another option would be the mimio devices but they are much more expensive.
To do a larger touch screen that could come out would be a different challenge because of the size to show it. This is the first solution I found for something like it that would be awesome maybe if it was built into the side of a SUV(possibly replacing the windows)
I really hope you arenot doing this whole build in Indiana.. I would love to get my hands on it!!!