I've been thinking about this for some time...
I am pretty sure the shortest path to a solution would be with Adventure Trailers, and getting their habitat married to the top of the truck.
When(/if) they release the the DIY kit I would experiment with it. It would be the perfect compliment for my shelving/sleeping platform in the rear, giving me a little more head room, maybe even standing (6'2").
EDIT: They currently offer a DIY kit but it doesn't have the pass through feature I'm interested in. Guessing on pricing, I'd say it'd be north of 8k to get them to do the custom (based on pricing for DIY kit & Taco Habitat)