Set title to a query var – how?

I’ve got a custom rewrite rule using a query var to intercept the main query. Is it possible to set the title to this query var (the ISBN-number) in some way? If needed I can modify the rewrite rule so that it uses a tag instead. Would that help? Any pointer in the right direction would make me very happy.

add_action( 'init', 'isbn_rewrite_init' );
function isbn_rewrite_init(){
    add_rewrite_rule(
        '^isbn/([0-9]{13})$',
        'index.php?isbn=$matches[1]',
        'top' );
}
add_filter('query_vars', 'isbn_query_var');
function isbn_query_var( $query_vars ){
    $query_vars[] = 'isbn';
    return $query_vars;
}
add_action('pre_get_posts', 'include_isbn_queryvar');
function include_isbn_queryvar($query) {
  if (!is_admin() && $query->is_main_query()) {
    $isbn = $query->query_vars['isbn'];
    if (!empty($isbn)) {
      $query->set('meta_key', 'ISBN');
      $query->set('meta_value', $isbn);
    }
  }
}