In the meantime, this was a fun exploration of the GAE environment. It makes it clear to me that this environment is still a toy. But a very interesting and promising one.A more sophisticated but also much more complicated solution to the same problem has Peter Dolan: HTTPMR, a Map Reduce implementation in Python for running on HTTP based web clusters such as Google App Engine.
I am experimenting myself with a completely rewritten, not yet released version of ErlyComet, for adding a RESTful, highly scalable Comet layer with transactional cache in front of Google App Engine (or any other service which provides easy, cheap, reliable and scalable solution for persistence and shortlived HTTP requests)