So what does an extension developer who wants to keep up with trunk builds do? Use my handy address book rewriting branch. If your extension works on the tip of the tree or its various branches, it will likely work on the trunk when the patch is committed.
The repo is also world-writable (as long as you have access to hg.mozilla.org), which means that anyone else with in-progress Address Book rewriting patches can push there too. Don't worry about creating new heads, I'll sort all of that out when I see changes to the repo.
Finally, I would like to point people to the tentative roadmap to the rewrite.