They can, for example, have a policy that every code change is reviewed by a non-US employee.
You'd still have to run that same algorithm against all distribution mirrors, and even then, you'd have to find a way for any suspect distribution site not to be able to detect who's downloading (giving a potential reviewer a "good" build, while giving everyone else a false one).
Come to think of it, this might be a good case for distribution over bittorrent?