We tried to do that with http://sopablackout.org. Granted, this drop shadow implementation will get people's attention more, but we wanted to focus on allowing people to learn why the site is blocked.
<? if(date("Ymd")=="20120118") echo '<script type="text/javascript" src="hxxp//js.sopablackout.org/sopablackout.js"></script>'; ?>
Note: replace hxxp with http.
Preview of an actual page with the overlay: http://www.nitrogenlogic.com/sopaindex.html