Thanks @jasonbrooks & @dustymabe. Wow. That is just great.
Not mentioned in the link but kind of obvious for non-standard packages:
- Add your custom rpm repo defintion to: /etc/yum.repos.d/your-custom.repo
- Make sure the rpms install to rpm-ostree supported locations (e.g. not /usr/local)
- Add any public GPG keys for the repo.
I have some re-packaging to attempt before I can say these bullets cover all the additional steps but it sounds like that would be it.