I would think a very large spool of fine cable would be a terrific idea, with some kind of braking descender running down it. They make similar things for wall-climbing. And why not giant Kevlar trampolines acting as sunshades about half way up and every twenty floors? Emergency baloons on the roof that would inflate and liftoff with the pull of a rope? Inflatable glide-discs that you could ride like a Frisbee? I dunno. It is madness to build such a trap with no alternative egress.

A