Trac 0.12 includes a plugin[1] to scan commit messages and automatically add comments and close tickets based on keywords in the message. This proces has to be triggered by hand, so it seems useful to include a bzr hook with trac-bzr that does so.
Attached is a proposed hook script that works for simple and shared repositories. The script uses the tip change hook, so it also works on updates and pushes.
The script also registers a command 'trac-project' to easily print and set the trac_project property in branch.conf file. It is not possible to set the trac_project property on a repository. So in a shared repository setup, the property has to be set on every branch. A warning is issued to inform the user about this.
The script could be installed in /usr/share/pyshared/bzrlib/plugins/trac/__init__.py to make it available systemwide as soon as someone installs trac-bzr. I don't know if easy_install and eggs allow all that.
If the script is invoked within a smartserver (e.g. commit to bzr+ssh:// location and the plugin is installed on the remote site), the script fails, complaining about accessing a location outside the server jail. I don't know yet how this can be fixed. I tried calling the trac-admin command by hand, but since I cannot get the local repos url (it gives /) that also failed for now.
[1] http://trac.edgewall.org/wiki/CommitTicketUpdater
[2] http://trac.edgewall.org/wiki/TracRepositoryAdmin#ExplicitSync
Just to note that the hook in trace 0.12 ignores bzr revision properties, so you can't use things like "bzr commit --fixes " with it.