Nope, instead i have a "update-code.sh" which is being called from a new link in the APM to update to the latest code of the master package. This would in turn update all submodules to the corresponding versions.
In theory though adding the link to check out the master for each submodule should be fairly easy, yet it isn't really the switch from tar checkout to git checkout that is mentioned above. It would require a git submodule to be in place.
I only use branches to be honest as I import the tags from CVS (the code which moves CVS code to git adds branches and TAGS) and I don't need additional tags for my workflow, especially if you work with submodules and point to actual commits for each package.
As for subtree, I actually started with subtree (and still have a version with subtree in place). Yet I ran into issues with custom packages with client specific code and the main repository which contains all the subtrees and keep that in sync with the various branches for development, testing and production. Ultimately I gave up and switched to submodules, never looking back. But I do have code which transforms ]project-open[ and OpenACS into ONE repository using subtree as well (if that is of interest).
