Where would I put something like:
this.userProxy = this.facade.retrieveProxy(app.model.proxy.UserProxy.NAME);
You could do this in onRegister(), however if you do this, you are making a requirement on the order in which you register your Proxys, since UserProxy needs to be registered before the Proxy that's trying to retrieve UserProxy. That's not so super, because, while you can leave a note in the comments about how important it is that the order of Proxy registrations remain the same, it isn't *enforced*.
A better way to handle this is to pass the UserProxy into any other Proxy that needs it as a constructor argument. That forces you to have constructed the UserProxy first.