I wondered about the same thing before we got our van. It just wouldn't work the way ours is laid out. I don't know how you'd attach a hook or eye bolt, and to what. I have been working on how to hang some kid size beds down from the rails of the penthouse.
Off topic, but my oldest son does usually sleep in a
Hennessy Hammock and a
Big Agnes sleeping bag. He says it's the most comfortable he's ever been while camping. He really loves the combination. The hammocks are well design, you sleep in them at an angle so you're not sleeping with a bent spine. The nice part about the Big Agnes bags is that the mattress is the bottom of the bag so theres no rolling off.