I tried to ask about this over on the BuddyPress forum but no luck there.
I'd like to make the "Moderator" role work in identical fashion to how "Private Groups" work for ordinary members.
So, you'd have a public group and users could join without any processing. However, there'd be a "Join Moderators" option, letting members apply to become moderators, which would add them to the queue for the existing Group Admins/Mods to review.
Do you think that could fit into your plugin's scope?