Sorry, answering my own question with an (obvious) answer. In case anyone else is interested:
function my_relationship_query( $args, $field, $post_id ) {
$args['orderby'] = 'date';
$args['order'] = 'DESC';
$args['post_status'] = 'publish';
return $args;
}
add_filter('acf/fields/relationship/query', 'my_relationship_query', 10, 3);