Personally I think mirror-on-a-stick would be a waste of development time (just lean, doofus), but wouldn't it be relatively simple to have an inventory item that, when "used", temporarily moves the player eye to some point on a held object? A handheld scouting orb, basically. Actually, now that I think about it, a periscope would be damn handy. Scouting out areas above and below the player has always been a weakness in Thief.