I mostly use pygobject when handling our flatpak repos. Unfortunately, I still have to fork flatpak for build-update-repo. It would be nice if there was an API equivalent of this.
Taking a quick look, I think just if flatpak_repo_generate_appstream and flatpak_repo_update were available, that would cover most of it. Certainly it would be nice to get all the static delta handling, but I personally always handle that separately, anyway. Hmm, now that I look, it appears that none of the repo operations are exposed in the API.