Note that I added a group refresh to pick up the 'hard-adds'. I think the correct place to deal with this is in fact to alter the GroupContact Add functionality - ie.

IF (CRM_Core_DAO::executeQuery('SELECT id FROM civicrm_group_contact_cache WHERE group_id = 1 LIMIT 1)) {
  // Add the contact just added to the group to the group_contact_cache table.
}

I started that a bit but I think it should be dealt with separately after this is resolved