Based on communicating with GitHub's support team, I found that GitHub currently offers no direct mechanism for a user to mirror repositories in this fashion.
However, one can ask GitHub to install this service for repositories which are part of an organization. GitHub then configures an existing repository as such a mirror and pulls from it in an interval that is a function of the number of overall mirrors they have.
EDIT: as Stuart points out, GitHub no longer accepts requests for mirroring arbitrary repositories. The only remaining option is the solution I posted in my question, i.e., creating a post-receive hook to automatically push to your GitHub repository.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…