Hmm, now I'm not sure I like this idea. Scrolling to a post to click the tag seems not necessarily appropriate since the tag relates to the user and not the post.
The idea is that it's slightly inconvenient having to type the username. But I guess that's what auto-complete is for.
It looks that even here we already using @ Tag instead of quote, it looks a more clean way to discuss if we want to discuss something but we also want to reply to other X person without having several quote bars.
Adding the tag add-on to my site made a massive difference to block quoting and quoting to alert someone.
I even had to tell people to stop doing it as there was more quote content than actual content on every page.
So having Tag next to Reply may help even more with that.
What would say not to have another thing in thread but use the little nickname under the post for that? It would be inconvenient at first as anything new but users will learn it.
It's still not really consistent for the same reasons mentioned in the other thread. Usernames always link to the member's profile. That shouldn't change.
All the other links in that area are used as controls. Therefore it completely fits to have an additional link next to "Reply" which is simply "Tag".
Actually, I just tried a few scenarios related to this and I don't think it will work in the majority of cases.
Rarely do you tag someone at the start of a post so you either click Tag and reposition the cursor in front of it and start typing, or, you start typing, scroll up, find the post/user, click Tag, scroll back down again, and resume typing.
It's going to be quicker just to type @... in most cases I suspect.