Seems like an easy bbCode mod?

[kh99]

I don't know if it exists or not, but the reason it's not so easy to do with bbcodes is that posts are cached after the bbcodes are processed, so you'd need some way to prevent a post from being cached if it contained a "you" bbcode.

I haven't tried it, but you might be able to cheat by creating a plugin using hook postbit_display_start and code like:

$post['message'] = str_ireplace('[you]', htmlspecialchars_uni($this->registry->userinfo['username']), $post['message']);