From a technological standpoint, it's easy to pin the channel. It's as simple as creating a file like this in your project: https://github.com/catern/nix-utils/blob/master/pkgs.nix

And importing that file whenever you want to use packages from nixpkgs: https://github.com/catern/nix-utils/blob/master/default.nix

If you want to have a separate channel which you can update on your own, just fork nixpkgs on Github and point your pkgs.nix at your fork.

The Nix binary cache keeps every version of every binary in nixpkgs that's ever been built on the Nix project's build servers. (Note that that's not absolutely everything in nixpkgs, since some things aren't built centrally on the build servers for various reasons, but it's most things.)

However, I might be misunderstanding. Was there some other feature or ergonomics benefit that you were looking for?