@splitbrain wrote a blog-post in 2014 which featured his version of a backup-solution using a rpi and some scanner. All of the credit belongs to him.
Meanwhile, flash-forward 2017 some of the software has progressed significantly, so when I took up this adventure I used his codebase to implement my solution. So feel free to have a look around. I wrote some additional notes in this Gist on how to set it all up.
@slpitbrain didn't include a licence so I won't either.