Nice! Could you open an issue on https://github.com/jupyterhub/binderhub with the git ls-remote idea?
We could already add this to the “bare git repo” provider https://github.com/jupyterhub/binderhub/blob/0b8f99af0a23a7828810b03f010edcefc9a46195/binderhub/repoproviders.py#L126-L160