I can see a fairly simple extension assuming a friend-of-a-friend style forwarding. It would be that much slower than one-on-one and participants have to remain online for the chat not to experience splits.
As soon as the thing needs to work with participants joining/leaving at will you'd need some kind of supernode or hidden server and that's a vulnerability.
Distributed databases have solved both the problem of splits, and requiring a fixed master server. MongoDB for example has a master node, but if this leaves the other nodes will elect a new one. All nodes accept queries and will sync up over time.
As soon as the thing needs to work with participants joining/leaving at will you'd need some kind of supernode or hidden server and that's a vulnerability.