Super great write up. This really took me back to my days of working on the container platform behind CloudFoundry.
In particular, we also used to use loop devices [1] but with AUFS mounted on them.
Later we moved over to BTRFS and then Overlay on XFS [2] to help with our unprivileged (security) story.
Also, this was a great piece of technical writing. Thanks for sharing!
1: https://github.com/cloudfoundry-attic/garden-shed/blob/6c5b0...