The point here is, that user B can't checkpoint his work at all. He will end up doing ZIP-backups to save his snapshots.
Git to the rescue! ...Or maybe not, because somebody is bound to get annoyed enough to just force-push his changes :lol:
Anyways, maybe we might be optimising prematurely here... How many people are even interested working on this, and what are their skills? In my experience, the number of contributors in any project is rather limited. Might be enough to just open a social group, and appoint somebody to do general coordination. A pull-based system might be overkill for the coordinator, or it might eventually turn out to be necessary, but somehow I doubt it.