The blessed transaction log could be put on GitHub, it only adds records to the end.  For more formal and general backup, it would be easy to set up a minimum size 500GB 108 US$/year rsync.net account with the keys to the kingdom being spread out to e.g. Arthur and whomever else makes sense.  Very easy to use, ssh, scp, and rsync of course, can even host git repositories, albeit without any frills.  There's also a FreeBSD ZFS snapshot facility that by default does the last 7 days.