AIUI, this is a trust issue. The problem is that people are worried about depending on a branch owned by someone else. For mirrored branches, the need for this dependency is a little forced—there's no a priori reason for forbidding multiple Launchpad entries for a single Bazaar branch.
Subscribing yourself to the branch doesn't address the trust issue. Writing on the whiteboard is definitely a step, but it's not sufficient. The solution mpt describes would probably suffice, but it would be a lot of work to do well. Another variation is to allow projects to say that they explicitly depend on "branch X", and then lock branches depended on. Another solution might be to allow branches to be registered as "hardlinks" to other branches, when the URL changes, the link is broken.
Allowing multiple registrations of a single branch is another interesting solution. What are the issues in doing this?