Any fetch-through caching solution (like an nginx with some caching config even), deployed at a site doing a lot of requests, and that endpoint configured on each machine should help reducing the number of requests for the same contents. We could help providing example configs, as well as documentation on how to test it is working - in addition to ensuring failure modes in Nix are well-understood.
Anything doing something more fancy than that “closer to the client” becomes a bit of a liability on our side, as we need to be aware of such deployments when changing anything in the path. Currently our only stable public interface exposed by cache.nixos.org is the http protocol, we need to be very careful before encouraging to rely on something else, as that’ll then also become some public API which would need to be supported for a considerable amount of time.