Add maxlength to Privatemsg module's message body field

Hi, I'd like to know if it's possible to somehow add maxlength integration for the Privatemsg module's body field.

I tried adding the following code in privatemsg.pages.inc:

  $form['body'] = array(
    '#type'               => 'text_format',
    '#title'              => t('Message'),
    '#rows'               => 6,
    '#weight'             => -3,
    '#resizable'          => TRUE,
    '#format'             => isset($format) ? $format : NULL,
    '#after_build'        => array('privatemsg_check_format_access'),
    // Here are my additions
    '#maxlength_js' => 'TRUE',
    '#maxlength' => '100',
    '#attributes'         => array(
      'maxlength_js_label' => array('Content limited to @limit characters, remaining: <strong>@remaining</strong>')),
  );

I arrived at this code by using dpm() on a different textarea for which I had enabled maxlength.

I attached a dpm() of the resulting output. However, this doesn't work.

Is there a way to use maxlength with the Privatemsg body field?