I tried to do this (a centralized Java-based web socket servlet) and failed, because the Construct is inherently a peer-to-peer architecture. The only way you can make it work (short of writing a whole new multiplayer plugin) is to have your "host peer" as a browser running on your web-server.
Barring that, you would have to create a completely new Multiplayer plugin that runs on an external customizable API.
I am going to do this eventually, but it requires more time than I have at the moment.
If you do create such a plugin, put it on Github so that other folk can help develop it. I would also recommend using http://socket.io/
See these posts for reference: