Hum, that might be cool, but the 40 is a quite a bit to short to sleep in. Although I've had ideas for a long time about some kind of platform that would be the same length as the flat part of the tub, then slide out and drop down to butt up against the end of the tub with the tailgate open, then you could have some kind of canvas skirt go around the platform to the lift hatch and barn doors to close it all in, it would take some expementing, but I've always thought something like that would be sweet.
Funny you mention that, my dad was telling me yesterday that he saw a 40 turned into a little camper back in the day and thought it was way cool.
Drew (amaurer) did that...had it at SnT a couple years ago, and I think there's a thread in the 40 section.
Saw that straightsixstraightsouth rig on Facebook on Mark Whatley's page (Cruisers Only), looked nicely set up. I've also had ideas of expo'ing in my FJ40, as stated above much easier with 1 than with 2.