Let\'s say I want to use Google GWT on the client side and Google AppEngine Python on the server side. Furthermore, I want to be able to use RPC calls to the server as well
You can do it with the help of a tiny extra service (I host one for free):
http://www.brightyellowcow.com/blog/Comet-AJAX-push-using-PHP-or-Google-App-Engine.html
The service's sole job is to provide a wake up to the client application, and it does this using a long polled ajax request. Your App engine application pokes the service via http when there is new data uploaded, which causes the client to wake up and go back to the app engine application for the new data.