How to add a META tag in head conditionally to entity content
It's definitely possible, you could do this using hook_entity_view() for example:

function MYMODULE_entity_view($entity, $type, $view_mode, $langcode) {
  if ($type == 'my_entity_type' &amp;&amp; $view_mode == 'some_view_mode') {
    // Check the field items.
    if ($items = field_get_items('my_entity_type', $entity, 'field_visibility')) {
      if ($items[0]['value'] == 'private') {
        $element = array(
          '#tag' =&gt; 'meta',
          '#attributes' =&gt; array(
            'name' =&gt; 'robots',
            'content' =&gt; 'noindex',
          ),
        );
        drupal_add_html_head($element, 'robots');
      }
    }
  }
}