Configuring coreos-assembler to use a container registry as rpm-ostree base (or: avoiding "rebase")

I am diving into the world of CoreOS assembly, primarily following this helpful (and admittedly dated) article.

I’m now trying to replicate section 8 - “Ship images with updates enabled” where the default ostree remote fedora.conf (now fedora-compose.conf, as it seems to me) is first removed and then another remote pointing to a static file server which holds the ostree data is employed (by placing a new remote .conf into the corresponding overlay.d subdirectory).

Nowadays, coreos-assembler generates an .ociarchive file which I can push to a container registry. Then, I would use rpm-ostree rebase ostree-unverified-registry:... to rebase any running system to follow that registry.

How would I, however, build CoreOS to follow the registry firsthand?

What would the remotes.d configuration file for that look like? Is it even possible to bake the OCI rebasing into the build at this moment? I’d prefer not to rebase through Ignition on my client machines.