I don't know about the specific features you requested, but the Copper mod (among many other things) improves coop. You can see the changes here:
The big ones for the purposes of a ludicrous event like this one are the telefrag protection for spawnpoints and teleporters and the way that elevators now wait for nearby players. Still 64 sounds like it might just be pushing it...
Speaking of Copper, several episodes have been released for it, and they're all designed to work well in coop. Of those, I'd say smej 1 and 2 are probably the most likely to survive with so many players. I'm sure other people more experienced with coop maps would be able to suggest packs fit for such a specific purpose.