This is really cool, but it’s very Btrfs-heavy. I would give extra nerd points for making Windows run on Ext4, XFS or ZFS. We already have https://github.com/openzfsonwindows (great avatar) so who knows? :)
Heh, I’d like to see NTFS support. At that point it can be a drop in replacement for winload, and from there, it can be taken in many different directions.
Another thought is to access files from a redirector or hypervisor so that a Windows OS can boot where its files aren’t owned by a local block device at all, similar to DosBox. This in turn opens the door to sharing files across a fleet of Windows VMs, so each VM only needs to record different files. Taken to an extreme, the storage cost of an instance could be as small as a copy of the registry, even after updates are applied.