After all I tried It was not possible to get mercurial working with hg-git against a Gerrit (http://code.google.com/p/gerrit/) server repository. It's possible to pull from the server, but pushing is not allowed on the same branch as the pull from ref/heads/master.
Push must use refs/for/master as target which is the way Gerrit runs it's incoming changes for reviews. see http://gerrit-documentation.googlecode.com/svn/Documentation/2.4.2/user-upload.html#_git_push
How can I specify the git-branch to be used in git push command within hg-git?