How do I exclude a single menu item from the sitemap?
Maybe you want to consider the approach described in comment # 1 in issue # 985138 (for D7), which is like so:

/**
* Override site_map function
*/
function hide_in_sitemap_preprocess_site_map(&amp;$variables) {
  $mids = variable_get('site_map_show_menus', array());
  $output = '';
  if ($mids) {
    foreach ($mids as $mid) {
      $menu = menu_load($mid);
      // Use menu_tree_all_data to retrieve the expanded tree.
      $tree = menu_tree_all_data($mid);
        foreach ($tree as $branch =&gt; $value) {
          if ($value["link"]["options"]["hide_in_sitemap"]) {
            unset($tree[$branch]);
        }
      }

      if (module_exists('i18nmenu')) {
        i18nmenu_localize_tree($tree);
      }
      $menu_display = _site_map_menu_tree_output($tree);
      $rendered = drupal_render($menu_display);

      if (!empty($menu_display)) {
        $title = $menu['title'];
          $vars = array( 'title' =&gt; $title,
                        'content' =&gt; $rendered);
        $output .= theme('site_map_box', $vars);
      }
    }
  }
  $variables["menus"] = $output;
}