These projects are mainly side projects, or small freelance jobs I've done through the years, and I want to save them for posterity; who knows when I'll need them, right?
My solution to this problem is called tar. Just clone the Git repos to /tmp, tar up the results, and push the tarball to S3. Done. Archived for posterity for a few cents per month.
This isn't SVN anymore: It doesn't take special voodoo to host a repo. If nobody needs to push or pull from a repo, tar it up and archive it. If one person needs to push and pull from a repo, store it on that person's local hard drive (with backups, of course). If two people need to push and pull from a repo... Well, this is no longer an archive for posterity, this is an active project, and can the team really not afford to pay Github something like $5 per month per repo?