Workbench Access Support

Our site uses Workbench Access to create editorial section control. I'd like to theme the site based on the current section, so have written this module for themekey which queries the workbench access id of the current node. I'm wary this approach is slowing site performance considerably.

Have I got the right idea, or is a different approach needed?

function themekey_workbench_themekey_properties() {

$attributes = array();
$attributes['workbench:aid'] = array(
    'description' => t('Workbench: Access ID - The id of the of the taxonomy term used as the access id'),
    'validator' => 'themekey_validator_ctype_digit',
    'page cache' => THEMEKEY_PAGECACHE_SUPPORTED,
  );

$maps = array();

 $maps[] = array(
    'src' => 'node:nid',
    'dst' => 'workbench:aid',
    'callback' => 'themekey_taxonomy_nid2_wban_aid',
  );


return array('attributes' => $attributes, 'maps' => $maps);
}



function themekey_taxonomy_nid2_wban_aid($nid) {
  $aids = array();

  $query = db_select('workbench_access_node', 'wban');
  $query->addField('wban', 'access_id');
  $query->condition('wban.nid', $nid);
  $result = $query->execute();

  if ($result->rowCount()) {
    foreach ($result as $item) {
      $aids[] = $item->access_id;
    }
  }
  
  return count($aids) ? $aids : NULL;
}