I have no idea why I didn't install this sooner. I guess I thought users would abuse this and tag the mods all the time. Not so- only usergroups with permission, can tag usergroups set as taggable. Works great, finally I can tag all mods or admins without going down the list!
Brilliant extra functionality to the now core tagging system. Only suggestion would be to add a rebuild button to the ACP to automatically run the query in post #1590, which enables email notifications to all existing members upon being tagged.