You can install nix-serve on any machine to share its binary cache (including patched applications). Then add all of the enabled machines to every machine's nix.binaryCaches in order of preference. If they don't have what you need, it will fall back to the upstream server. You'll want to make sure they are upgraded in an appropriate order to minimize the need to leave your network for updates. It's true this isn't well documented, but I figured it out in about a day and was surprised at how simple it turned out to be.

You can even go further and set up your own build server to service all the architectures you need to support.