How to carry Xtracycle by car?
If the length of the bike is makes it legal, hitch rack with the bike sticking out would be the cheapest and easiest - You would be no wider than trailers and trucks, but care would be need to remember that. Get a rack fabricated that holds the bike vertically off the hitch, Strop the top roof the rack back to the roof bars. For ideas have a look at here and talk to an engineering shop.

Have a look for ideas to carry tandems - Here is one idea that would work.