Clone the homebrew repo. Use a cronjob or Jenkins or some such to periodically fetch updates to a server under your control. Modify homebrew and its setup script to use your repo (it's been a while since I did this but the Github URL for the homebrew repo used to be hard coded in one of homebrew's ruby scripts). Whenever setting up a new machine, use the modified setup script.
Periodically you can merge upstream into your clone to get updated recipes. (e.g., the aforementioned cronjob can mirror master from github to remotes/upstream/master in your repo, then separately you merge that into your master).
Your servers are now immune to github outages[1] and you can review recipe updates before your servers update to those recipes.
[1] unless of course the recipes you're using are hosted on github. But you could recursively mirror those sources too and modify the recipes as needed.
edit: I'm on a mobile device but I can provide further details the next time I'm in front of a keyboard in case this wasn't clear.