While playing around with Buckminster I read about the p2.mirror-Ant task. That's excatly what I am looking for: merging a new release into a p2-enabled update site. A great pity that it took so long to find it.
I played around with it on my test update site and everthing worked fine.
Perhaps p2-frustration will come to an end...
... and for the Germans: Kaum macht man's richtig schon geht's ;-)