No useful ideas to add if a bigger bed isn't an option but your post did bring back some great group sleep memories. Like the first time my boyfriend tried to stay the whole night in our bed, I woke up so sad that he had left. He insists that he woke up on the very edge of the bed, and right at that moment I kicked him off the rest of the way. One of the most useful bits of poly knowledge I've ever read was the suggestion of multiple covers. It seems so obvious now but I hadn't figured it out on my own. I'm sure the concept is over-rated but I enjoy trying on occasion for the comedy.
How about an air mattress for the odd man out? It's not great but it beats the floor.