Update for https://gist.github.com/wimleers/5353521.

edit.module

  print '<hr>';

  // Node -> EntityFormController
  $entity = entity_load('node', 1);
  $entity = $entity->getBCEntity();
  var_dump($entity->getTranslation('und', FALSE)->get('field_tags')->getValue());
  $form = array();
  $form_state = array();
  $options = array(
    'field_name' =>  'field_tags'
  );
  xdebug_break();
  field_attach_form($entity, $form, $form_state, 'und', $options);
  drupal_prepare_form('form-id-here', $form, $form_state);
  drupal_process_form('form-id-here', $form, $form_state);
  var_dump($form['field_tags']['und']['#default_value']);
  var_dump($form['field_tags']['und']['#value']);
  print drupal_render($form);

  print '<hr>';

  // CustomBlock -> EntityFormControllerNG
  $entity = entity_load('custom_block', 1);
  $entity = $entity->getBCEntity();
  var_dump($entity->getTranslation('und', FALSE)->get('block_body')->getValue());
  $form = array();
  $form_state = array();
  $options = array(
    'field_name' =>  'block_body'
  );
  field_attach_form($entity, $form, $form_state, 'und', $options);
  drupal_prepare_form('form-id-here', $form, $form_state);
  drupal_process_form('form-id-here', $form, $form_state);
  var_dump($form['block_body']['und'][0]['value']['#default_value']);
  var_dump($form['block_body']['und'][0]['value']['#value']);
  print drupal_render($form);
  exit;