I think that this would be an ideal way to implement a multiplayer coop mode in xonotic. It is true that you need new maps, but these would be rather simple, as they would only be "2.5D". I see the point in making "new maps for Alien Swarm", but Alien Swarm is not OpenSource. Alien Swarm is Steam junk, so it sucks. Xonotic is OpenSource, and therefore Xonotic rocks. There would be no alterations to the engine needed, because DarkPlaces already looks better than that Unreal engine they use in Alien Swarm. I don't think that this would be too far off the original game mechanics. Bunny hopping would be inappropriate in 2.5D though.
"If your killer instincts are not clean and strong you will hesitate at the moment of truth. You will not kill. You will become dead marines and then you will be in a world of shit, because marines are not allowed to die without permission. Do you maggots understand?" - Gunnery Sergeant Hartman