I was trying to solve this and made an example with a inner and outer container.
The idea is sound but there seams to be a drawing/timing issue. This concept work pretty good under Flash but is having issues here. Maybe RelayoutIfNecessary is not working properly? Anybody?
Keyhole.hype.zip (569,7 KB)
Update: Fixed using mutation observer (but omitting IE 9-10):
Keyhole_MutationObserver.hype.zip (570,6 KB) (Thanks for the hint jonathan)
But there is a polyfill https://github.com/webmodules/mutation-observer