What I thought about doing, is creating a unique service key for each operation and then passing it to the neworderservice so it knows which orders to look for but I haven't been able to figure out how to pass a unique key to it.

 

You can pass client-specific arguments (in the last parameter clientArgs shown below) when registering the callback:

 

        public void RegisterCallback(ServerNotifyDelegate serverDelegate, ClientNotifyDelegate clientDelegate,

                                                     Object userToken, params Object[] clientArgs)

 

These clientArgs are stored with the NotificationSubscriber, which is available to the service.  So for that sample, you could do something like this to get the arguments for each subscriber:

        foreach (var subscriber in __notificationManager.GetSubscribers(__serviceKey)) {
          var args = subscriber.ClientArguments;
        }

Hope this helps!