I'm sorry, the mirror is actually for sources, not build artifacts. I've updated my comment to clarify.
https://github.com/NixOS/nixpkgs/blob/733151b7825ee8840f7437...
Edit: It was apparently added in 2007:
https://nixos.org/releases/nixpkgs/nixpkgs-0.11/release-note...
Unless there is some magic that I'm missing, I don't think this option helps for our use case.