Well, MyBB supports such a function, plus a ton others that are all awesome forum add-ons, and MyBB is only under a megabyte large.. Maybe, maybe... It's not stupid, but not quite useful and epic as well, but it'd add so much to the forum.
It's been suggested, and declined. If people could see when Mods are online, then they could spam whenever there isn't one online. I can't remember why it was shot down even with Mods being listed as invisible, but oh well. Don't see it happening.