I'm afraid this would be abused. It would be good if the user/client could choose to tag themselves. This would be opaque to the OP to follow a thread privately; but maybe the OP could see metrics on how many have tagged themselves. Thoughts? or are we talking the same thing? 

 I was thinking it would be useful to post a note that you want to be sure someone sees but not have to put their handle directly into it. The tag would appear in the metadata and would be visible, similar to how replies work.