Just checked this, it does work but..
The top, bottom, left and right q-edge (window edges) do not display when this is added, the corner images show ok though on the window..
Im using it like this when the q has loaded if this helps..:
document.observe('q:loaded', function() {
Q.addCss("div.q-window div.q-center", "overflow:hidden;");
});
For the moment ive left it in: win.center.setStyle({overflow:'hidden',etc inside q.window.js
thanks
Andy