this seems a little needlessly convoluted. Suggestion:
```PHP

// base class
protected function _formatResponse($action,$result, $format){
     return json_encode($result);
}

// subclass
protected function _formatResponse($action,$result,$format){
   switch($format){
        case 'xml':
         // bleah bleah
         break;
       default:
            parent::_formatResponse($action, $result, $format);
   }
}
```