The way we modify the raspberry pi image without needing root is with a docker/podman container that uses libguestfs to mount the image and copy in our files.
The docker/podman container is based off of Uwe Dauernheim's docker-guestfs image.
podman build -t z2mpi .
podman run -v $PWD:/root/src:Z --rm z2mpi:latest
podman run --rm -it -v $PWD:/mnt:Z guestfs --format=raw -a /mnt/2022-01-28-raspios-bullseye-arm64-lite.img -m /dev/sda2 -v
Make sure to mount the image with the :Z flag if you're using SELinux, or you'll get cryptic errors.