As of 7.22, Drupal core supports attaching a field for a single form in the following form:

field_attach_form('my_entity_type', $entity, $form, $form_state, NULL, array('field_name' => 'field_myfield'));

The change record: https://drupal.org/node/1825844.

The commit: http://drupalcode.org/project/drupal.git/commitdiff/b9d7b6f0fca4266a4de5....