See this https://github.com/ondrejbudai/fedora-bootc-raspi/blob/cbff66a8bdb82b6276814765ffdb82c4ec4efa9c/bootupctl-shim#L7

Basically today the bootloader update payload is often part of the base image, but we need to support dynamically extending it with layered image content as well.

In the short term, what may suffice is to offer an explicit user-hookable phase that can copy things from the container to /boot as part of bootc install to-filesystem.