Wouldn't Git's branch feature provide the structure you need? You would have a Community Branch and a Follow Tutorial branch. The Follow Tutorial branch would only have commits by Casey, and then the Community branch would be able to pull in pull requests. This doesn't have to in the origin branch, either, the canonical Casey branch could have just the Tutorial branch, and anyone that wanted to host a fork could rebase regularly from that branch to the Community branch, owned by an interested party.
This way, no matter repository you fork, you will always be able to get the latest version of the Tutorial.
Of course, I could be seeing the problem wrongly. Hopefully the idea is helpful.