How can I callback the client and expose a new Channel with instance context

Question

I'm making a WCF service with netTcpBinding which has a main lobby with multiple chatrooms which the clients can enter. The Lobby class implements ILobby as the service contract.

When a client wishes to enter a room I want to callback the client exposing a new Channel containing the InstanceContext for the room he just entered but after much searching I am doubting that this is possible.

For example on the Service side I might have

class Lobby : ILobby
{
    Dictionary<string, Chatroom> rooms;

    public void JoinRoom(string roomname)
    {
        if (rooms[roomname].TryEnter()) {}
    }
}

class ChatRoom : IChatRoom
{
    public bool TryEnter(string username)
    {
        ILobbyCallback callback =
            OperationContext.Current.GetCallbackChannel<ILobbyCallback>();
        // How do I do this next bit?
        callback.JoinedRoom(pass some instance context here);
        return true;
    }
}

On the client side callback method I want

public void JoinedRoom(InstanceContext for the room on the service side)
{
    // Create a new WCF proxy using above InstanceContext
    // Create a WPF UI for the new room passing the proxy so it can communicate
    // with the room class directly without going via the root service
}

Is this possible? What's the best practice for spawning new classes with their own contracts on the service side? Or do I just have to bundle everything into one massive MyService class and handle everything myself?