Extract Drupal 7 form API elements from and existing entity form

example_entity_form_extract_element.php

/**
 * Extracts a form API element from and existing entity form.
 *
 * @param string $field_name
 *   The field name of the element to extract.
 * @param string $form_id
 *   The form ID of the entity form from which to extract the element.
 * @param string $entity_type
 *   The entity type fo the entity form.
 * @param string $bundle_name
 *   The bundle name within the entity from which to extract the element.
 * @param object $entity
 *   The entity from which the defaults will be derived.
 * @param string $langcode
 *   The language code that should be used for the element extraction.
 *
 * @return array
 *   A form API render array for the extracted element.
 */
function example_entity_form_extract_element($field_name, $form_id, $entity_type, $bundle_name, $entity = NULL, $langcode = LANGUAGE_NONE) {
  $form = array(
    '#parents' => array(),
  );

  $form_state = array(
    'build_info' => array(
      'form_id' => $form_id,
    ),
  );

  // Get default values for field if available.
  if (!empty($entity)) {
    $items = field_get_items($entity_type, $entity, $field_name);
  }
  else {
    $items = array();
  }

  // Get field and field instance information.
  $field = field_info_field($field_name);
  $instance = field_info_instance($entity_type, $field_name, $bundle_name);
  // Get the form element.
  $element = field_default_form($entity_type, $entity, $field, $instance, $langcode, $items, $form, $form_state);

  return $element;
}