It is considerably easier to use btrfs snapshots for a fallback instead of LVM in case system-upgrade goes south. (As indeed it did when upgrading my OG Pinebook.) It might not even need the “experts only” disclaimer. In the simplest form, you create a subvol snapshot and do a normal system-upgrade. To fallback, edit the grub entry for the previous kernel to use the snapshot. I could cover cloning the default grub entry with grubby for the fallback, but probably just exhort readers to practice editing grub entries at boot time.
If I may suggest something, it might be good to reconsider the title. As it stands it could be interpreted as using Btrfs to actually do the upgrade. I believe you are describing setting up a fallback or recovery in case something goes awry.
Please use comments on ticket #193 if you have questions or need to communicate with the editors about anything.