The best public list of domains I have found is the Project Sonar DNS (ANY) scans. I don't know how they do it, but their scans are pretty complete, at least for.dk domains, which are the ones I use.
But first, I definitely have to see if it's against GitHub's terms of service.
Almost anything that gets the name of a particular service bumped up to the top of someone's consciousness for a little while will shift some of those decisions toward that service.
This is why even the world's most popular brands (Apple, Coke, etc) never stop spending money on marketing / PR :)