ublock Origin already has a feature called "strict blocking" which does the same thing. In fact, the placeholder page that this extension takes you to looks almost exactly the same as the one that ublock origin uses.
Does that feature really send each URL you attempt to browse to a bunch of third-party services to be verified before it lets you browse them?
Edit: Made some not-very-thorough research and it seems like "strict blocking" in uBlock Origin does not do the same thing. It does local lookups against regularly updated filter lists. Like all of uBlock Origin's blocking, I guess.
Osprey is entirely asynchronous. It doesn't hold pages at all. If you visit an unsafe website, you might see it for 50-250ms before the connection is stopped.
