Render API callback: Hides the field value of 'text_format' elements.
To not break form processing and previews if a user does not have access to a stored text format, the expanded form elements in filter_process_format() are forced to take over the stored #default_values for 'value' and 'format'. However, to prevent the unfiltered, original #value from being displayed to the user, we replace it with a friendly notice here.
function filter_form_access_denied($element) {
$element['#value'] = t('This field has been disabled because you do not have sufficient permissions to edit it.');
return $element;
}