Add custom HTML to custom field
It turns out all I needed to do was to use hook_field_prepare_view() to add an extra, empty elements to the $items array

/**
 * Implements hook_field_prepare_view().
 */
function threejs_field_prepare_view($entity_type, $entities, $field, $instances, $langcode, &amp;$items)
{
    $items[1]['footer'] = array( );
}