amazon-ec2-ssh-utils should be fully functional now and I reached out to AWS EIC again to transfer ownership.

If you want to use it, I'd fork it since it's still being force-pushed occasionally. systems/nixosModules/test.nix has an example of how to set it up with the OpenSSH daemon (though it should be used without --source ec2-key-pairs to prevent man-in-the-middle attacks).