I think I had this problem already a while ago. But it seems to have reappeared:
I created a new MacHg document, pulled in a clone's directory from a finder window into MacHg's sidebar below REPOSITORIES.
When I now try to check which changes could be pushed to the parent repo by clicking on "Outgoing" or when I try to push, it turns out that the parent repo is not offered as a valid choice for the remote repo.
This is odd, since that clone has a valid entry in its .hg/hgrc!