panteLx yes it would require using the PHP extension API. Either from a new extension, or from the local extend.php if you just want to modify your own website.
I actually just had to deal with that in one of my premium extensions. I wanted an event post to be counted as last post when it's inserted in the discussion, but there's not really any way to do it. I ended up manually calling $discussion->setLastPost($post) from an event listener.
You could do something similar. Listen for new posts, check if it's the "tags changed" event post, then set it as the last post. It might show the person who changed the tag as the last person who posted though if doing that.
