Giving a default value to a password element in D7 form API?
You can try to add your default value directly with #attributes option just like that : 

$form['password'] = array(
  '#type' =&gt; 'password',
  '#title' =&gt; t('Password'),
  '#description' =&gt; t(''),
  '#attributes' =&gt; array('value' =&gt; $this-&gt;config['password']),
);


After I'm not sure if you will be able to get that value on validate and submitcallback, you should check.