Use token tree UI to show replacement tokens

The following seems to work in includes/token_help.inc:

  $form[$element_name]['help_text'] = _signup_build_token_help();
}

function _signup_build_token_help() {
  static $help = NULL;
  if ( empty($help) && module_exists('token') ) {
    $help = array(
      '#theme' => 'token_tree',
      '#token_types' => array('signup', 'node', 'global'),
    );
  }
  return $help;
}