Unsure if this is on scope for that backend or bootc in general, but usroverlay currently does not work on that backend due to it just being a wrapper over ostree admin unlock. Manually creating /run/ostree-booted gets it to try to run, but fails writing getting a lock on /ostree/lock. I've seen that there is some code on composefs-rs that is for implementing a transient overlay over a few directories, could that be used here?