I've been thinking about this since my last post, even discussing it with Seph (I know, I know, I need to get out more!), and it could be argued that if the guards were there to protect all past and future emperors, being above one individual and, in a way, above the emperor too, and they had ranks and regiments named after popular emperors, perhaps they could be without the apostrophe. You'd have to make it clear, though, as Hex says. And then we'll believe anything, cos it's your world.
(I think that's a different point from Gumboot's, although it amounts to the same thing I suppose.)