Drupal - Adding title attribute to #type password_confirm

The password confirm element is converted to two fields in a process function (form_process_password_confirm()), so adding your own process function to the password element would do the trick.

Per this issue it seems you actually need to overwrite the existing #process array instead of just appending to it, e.g for the user register form password element:

function MYMODULE_form_user_register_form_alter(&$form, &$form_state, $form_id) {
  $form['account']['pass']['#process'] = array(
    'form_process_password_confirm',
    'MYMODULE_password_confirm_process',
    'user_form_process_password_confirm'
  );
}

function MYMODULE_password_confirm_process($element) {
  $element['pass1']['#attributes']['title'] = 'Title';
  $element['pass2']['#attributes']['title'] = 'Title2';
  return $element;
}