is it possible to lock branch only OR change read lock on repository to write lock within one operation?

[Alexander Belchenko]

I'd like to have ability to change tags of some branch while I'm running qlog.

Unfortunately branch.lock_write() tries to lock_write() underlying repository, while I'm actually
don't need to write to repo. Unfortunately pack repos raise error, while knit repos works smoothly
in this situation:

Traceback (most recent call last):
  File "C:\work\Bazaar\plugins\qbzr\lib\log.py", line 883, in f_create_tag
    self.branch.tags.set_tag(tag_name, revision_id)
  File "bzrlib\tag.pyc", line 87, in set_tag
  File "bzrlib\branch.pyc", line 1363, in lock_write
  File "bzrlib\repofmt\pack_repo.pyc", line 1977, in lock_write
bzrlib.errors.ReadOnlyError: A write attempt was made in a read only transaction on
KnitPackRepository('file:///C:/work/Bazaar/qbzr-repo/.bzr/repository/')

Is it possible to lock_write only branch? If not -- is it possible to change read lock on repository
to write lock when needed?