I don't see the purpose of the please-use-existing-tags tag. As far as I know, the tags are meant to associate a post to certain topics represented by tags. In which case, I don't know why anyone would want to look up all posts with the please-use-existing-tags tag, especially when the nonexisting tags may have already been changed to existing tags in the post.
Is it possible instead to restrict users choice of tags rather than letting users have the ability to type in any tag whatsoever?