I think it's pretty clear in the tutorial that this is not possible. I've been playing with this myself and it seems the safest bet is going to be a dedicated host. To do that you are going to need a server that is always up, and have your host running the game on it all the time. This presents a few issues. Most notably what happens if someone else 'steals' the first connection and becomes host.
In the game I'm working on at the moment I get around that by changing the login screen in the multiplayer example to a full PHP login. If the intended host isn't logged in it just displays a 'waiting for server...' message.