What's the deal with the slave locked in the basement of the Argonian mission? He gives the usual spiel about needing "a key to unlock these bracers", but nobody in the place seems to have it.

Is this one of those slaves that are just window dressing, or is there a reason he's locked up down there?

I came across the same sort of thing in the smaller (not larger) of the plantations NE of Pelagiar. It drove me bonkers. I finally loaded up the editor and checked to see if it was possible to get a key and free the slaves... it wasn't. You'll probably find you're in the same position - there's no key.

If you're part of the Imperial Cult then you'll understand why he's down there. One of the quests assigned by the Cult involves the Argonian mission.