There is one big limitation in car-topable design nobody mention yet. It is weight of the boat. I believe there is a limit of how much you can put on the roof of the car, something like 85 kg. You will be hard pressed to design folding cartopable catamaran that light, unless it's inflatable <img src="http://www.catsailor.com/forums/images/graemlins/laugh.gif" alt="" />, or some very expencive exotic materials.

Inflatable car-topable catamaran, made in UK
http://www.catapultcats.com/