As it turns out, there is already a fix in EGit 1.1 for the unexpected EGit behavior I described here and here.
The new default for EGit 1.1 is to only push the active branch via Push to Upstream in case there is no explicit default spec. More is not possible at the moment due to some JGit protocol limitations. I can totally live with that, at least there are no surprises with local branches appearing in the remote repository silently any more.