Something like this is what I had desired when briefly experimenting with Fedora CoreOS and having to build layered images for ZFS support. I was new to CoreOS and was stuck right after I finished building an OCI image. Eventually I learned that the only way forward was to boot with a base image, then layer what I had built and run `rpm-ostree commit`.

I wonder if this project would've served my use case. The OCI images you build when layering FCOS images all build atop the base FCOS image. So I would expect them to be "bootable" in some sense.