Given how DC treated him there's maybe a slight hope he'd do it out of spite as a "one last hurrah."

I doubt it, but I wouldn't blame somebody for trying.

Alan Moore writing Fables to spite DC would be the comic event of the year. It would easily outsell anything on the market right now.

The implication is that Moore might potentially be able to pull the same move, releasing the watchmen, top 10, v characters into the public domain

Moore has a complicated relationship with Watchmen (and VfV), spurring from his complicated (and constantly changing) relationship with superhero comics. One of his bones of contention with DC is precisely that he didn't want the work to grow offshoots; I doubt he'd want the world to write them freely either.