About the only things I'd worry about are (1) paint and (2) derailers. For the paint, put a blanket over each bike -- easy.
You could remove all the derailers and tape them to the chainstay or seatstay to reduce the chance for damage, though that sounds like a lot of work. (Not as much work as fixing 20 bunged-up derailer hangers, though.) Alternatively, you might put a heavy cardboard box over each derailer, tape it securely in place, and then put it in the van and put the blanket over the bike.