BuddyPress.org

Okay, not a new hook per se, but a new admin menu group (BuddyPress) to which you should be able to add additional submenu pages. It still uses the add_action call to a WPMU hook (admin_menu).

Look at how its done in bp_core_add_admin_menu() in bp_core.php. Then, additional submenus under BuddyPress menu group are added in other BP components–like xprofile_add_admin_menu() in bp_xprofile.php.