Full custom functionality plugin example for: http://wordpress.org/support/topic/how-can-i-change-protected-params-xmlrpc

custom-functionality-plugin.php

<?php
/*
Plugin Name: Custom Functionality Plugin
Description: Custom functionality specific for this site, regardless of the theme used.
Version: 1.0
Author: Example
Author URI: http://example.com
*/


/* =Unprotect certain custom fields
------------------------------------------------------------ */
register_meta( 'post', '_mf_write_panel_id', 'custom_sanitize_as_is', 'custom_auth_always_allow');

function custom_sanitize_as_is( $meta_value, $meta_key, $meta_type ) {
  return $meta_value;
}

function custom_auth_always_allow( $allowed, $meta_key, $post_id, $user_id, $cap, $caps ) {
	return true;
}