So I can limit my app with UPNP NATs capable clients. I would love to see any Opensource Java project using GAE as server for keeping maps like (ClientID, IP:PORT) (with example of some on clients simple hole puncher) Is there any such project?
Why not just stay with http transport, which usually works well with firewalls (otherwise users complain).
As long as you have the clients call you in the cloud, it should work well.