I wrote the multiplex transport around 2006 and haven't looked at it much since then. It's certainly no longer supported.
That said, multiplex was designed to make it possible for a server to communicate asynchronously with a client without creating a ServerSocket on the client, to avoid firewall problems. If you use the virtual sockets and ServerSockets in multiplex, you might be able to do what you want.
I should add that the performance of a virtual connection, IIRC, was about 50% of the performance of a real socket connection.
If you want to read the docs and have questions, I might be able to help. Can't promise, though.
As long as you're thinking of an intermediate layer, why not put a single (real) ServerSocket on the client side of the firewall and distribute the messages from there to the clients?