Shoutbox

Again, people are slightly seeing this as overcomplicated.

Easy way to do it: Add a "admin_comment" field to the database which is retrieved at the same time as the rest of the data. As long as the admin who changes the user name remembers to update this field (I don't know what the MyBB control panel is like, and how easy/hard it'd be to add this field in there) then it'd work quite well.

Of course it's open to abuse, but mods/admins don't do that.