Unable to extend Orders Rest Api of Magento

I am trying to extend Orders rest api in magento. For e.g To create a new update method. For doing this, I have created a new class in the local copy: The file I am trying to override is : app\code\core\Mage\Sales\Model\Api2\Order.php

But when I try to access the new method then I get a "Resource method not implemented" error message. I believe, the rewrite does not work in this case.

Similarly, I attempted to rewrite products rest api but over there rewrite method works correctly.

Just for your reference, I have pasted the code for Update Orders Rest Api below.

1. \app\code\local\MyModulename\Sales\Model\Api2\Order.php

   class MyModulename_Sales_Model_Api2_Order extends Mage_Sales_Model_Api2_Order { protected function _update(array $data) { echo 'in update function'; } } 

2. \app\code\local\MyModulename\Sales\etc\api2.xml

3. app\code\local\MyModulename\Sales\etc\config.xml

Please note, this module is in active mode.

Any help would be appreciated. Thanks