Thanks—that sounds promising! Nice that it blocks search engines too. But that official guide make it sound like I'd need to send every past user a download key or a password? It doesn't mention pre=existing owners being an exception. (And it doesn't say whether pre=existing owners will still see it when browsing their Library.) Maybe the docs are just unclear? Don't want to find out the hard way!
(Ideally pre=existing owners owners would even see it when they do a search—I know that's how I always go to my games—but I doubt that's possible.)