Following improvements in the container-toolkit project this can now be directly rpm-ostree installed and so the referenced fork above is no longer needed and has been marked archived in GitLab and on the corresponding published GitLab Pages.
A new pull request has been accepted by the core NVIDIA driver project which includes updated documentation with new working ignition snippets regarding layering + some other changes that have occurred. FCOS kernel-specific driver images are still published to dockerhub as before until such time as the core NVIDIA project supports FCOS.