BuddyPress.org

Actually it is possible to bypass BP registration using the wp_insert_user() function.
It *should* be possible by overriding the /registration/register.php template file in your child theme. But all the good stuff like activating an account would be gone. However, you say you want to use WP’s registration system? Would require a BP core hack I believe.

Changing the BP_ROOT_BLOG wouldn’t do a thing since BP is activated network-wide.