Can hook_field_extra_fields be used with field collections?
The entity name for a Field Collection is actually field_collection_item and not field_collection (if you run var_dump(entity_get_info()) you'll see this).

This means your implementation should actually look as follows:

/**
 *  Implementation of hook_fields_extra_fields().
 */
function my_module_field_extra_fields() {
  $extra = array();

  $extra['field_collection_item']['my_field_collection']['display'] = array(
    'some_extra_field' =&gt; array(
      'label' =&gt; t('TESTING FIELD 123'),
      'description' =&gt; t('This is a test of hook_field_extra_fields on FCs'),
      'weight' =&gt; 100,
    ),
  );

  return $extra;
}


Also, dont forget your t()'s ;-)