Unfortunately, GitHub’s review request feature only supports requesting reviews of collaborators. Many of our maintainers are not collaborators, and so this won’t work for everybody. Perhaps the next improvement is using comments to mention them. The good news is the very hard part (identifying the maintainers) is solved.
Just FTR, you may request review of anyone who is part of the organization even if they do not have write-access. Maybe all maintainers (in particular those who don’t have write-access to the nixpkgs repo) should be added to a read-only team of the NixOS organization.