You seem like you know your way around BuddyPress registrations and group joining. Maybe you could find the time to fork this plugin, which is in high demand:
http://wordpress.org/extend/plugins/buddypress-auto-group-join/
As a new independent plugin or as bundled functionality in registration options is up to you I suppose. I'll be sure to donate if you make this happen!
http://wordpress.org/extend/plugins/bp-registration-options/
