IT'S HAPPENING
Entanglement, the "new" multiplayer mod for Boneworks, is getting proper automatic NPC syncing for the next update.
This means that Boneworks will be having a fully-functional multiplayer mod, with object sync and NPC/enemy sync. They even managed to sync physics properly by abusing joints, apparently.
The mod supports up to like 16 people in one lobby as well.
Boneworks does support custom maps but I'm thinking most of the campaign maps would be a bit of a clusterfuck with many people. Could be fun though