This will probably be a low priority, but I do think it would be awesome and helpful.
A branch most often has a specific purpose, such as fixing a bug, or implementing a feature. There are certain subtasks involved in the process, such as updating the change log or NEWS file, or documenting the code. If these tasks are not completed, chances are the branch won't be accepted for merger with it's target branch.
By associating tasks with branches, it will be easier to remember such things.
A use case could be that a contributor proposed to merge his new feature branch into the mainline of a project. The project maintainer reviews the proposal, but rejects it because it doesn't follow coding guidelines, lacks documentation and misses some unit tests. The maintainer then adds these tasks to the branch, so that the contributor can cross them off when they're dealt with.
We are actually working on something like this, but in a slightly different way.
We are implementing code reviews within launchpad. Code reviews will be attached to a merge proposal. Following though with you example above, the contributor proposes to merge his feature branch on mainline by creating a merge proposal for that branch targetting mainline. They'd set the status to "Needs review". Someone looks at this code and will be able to comment. A conversation will be connected to the merge proposal and viewable through launchpad. This conversation will be similar to bugs in that many people can comment and vote on the code. When the proposal has been approved, it will be signed off by someone with authority to land code on the mainline branch.