Injecting configuration files after installation, before first boot

How does one inject a configuration file post `bootc install` but before the first system boot? A customized `/etc/fstab` is a good example or perhaps some systemd mount units. With a non-ostree system when the installation chroot is setup this is a straightforward copy operation. `bootc` should document this process a bit better or provide a more convenient method as it will be a common need to drop some configuration files in place post install but before the system boots for the first time. Not everyone will want to adopt ignition to do these tasks.