The current bootc-base-imagectl script is quite helpful when generating custom bootc images from fedora rpms. It is a tool that is included with fedora bootc but does not exist upstream. Upstream bootc-dev seems to already have composefs-native backend working.
I’ve started experimenting with bcvk which does much of what I am aiming for with bootc-base-imagectl. Neither of them get me to producing my own base image from a pile of packages though.
The tool selection advice is quite helpful and points me to using bootc directly as the way to go. I have more to learn.