I almost have a solution here. With the seats turned toward each other as the photo at the top of the thread shows, a portable bunk solution like http://www.amazon.com/Disc-O-Bed-Cam.../dp/B00BGU41G2
would solve the problem. There is also a kid version called kid o bunk.
The problem is the bottom bunk is only 11 inches from the floor. We need at least 2 feet to clear the seats and have the lower bunk rest above the seats - as the bunk is about 10 inches wider than the between-the-seat area if that makes sense.
I'm thinking of contacting the manufacturer to see if they can provide an extension for the legs, or alternatively make something home made out of 6x6 blocks.