On a scale of 1-10, this registers a solid 7 on my personal rustle-meter. Especially since I have friends in Russia who do open source work on Github :(
But anyhow, just set up a mirror at BitBucket or Gitorious, and have them push to one of those for pull requests.
Logically I would think the comments are just an extra layer added by github, but theoritically git could have comments also decentralized and stored, this is why I'm asking.
github-backup is repository-focused. It does not try to back up other information from GitHub. In particular, social network stuff, like users who are following you, is not backed up.
github-backup does not log into GitHub, so it cannot backup private repositories.
Notes added to commits and lines of code don't get backed up yet. There is only recently API support for this.
The labels that can be added to issues and milestones are not backed up. Neither are the hooks. They could be, but don't seem important enough for the extra work involved. Yell if you need them.
You're right that it would be possible to also decentralize the comments, but that's not how GitHub does it, they just store those on their own, and there's no part of Git that is explicitly built for storing code review comments, you would need to add something on top using the "note" system or something similar.