I just tried it (Score 5, Informative)

It's a very, very evil hack. It works, for some definition of working - it'll make your Metacity very, very slow. It hooks into Metacity so that every time a window is exposed or does a redraw, it recalculates a thumbnail of the window.

This means dragging a window over multiple other windows will make the window manager unresponsive for quite some time! Anyway, hitting the magic button does produce a pretty thumbnail though.

This is definitely not useful in the real world, but still cute :-)