Extended version of http://www.nicklewis.org/node/967 recipe for partial nested form update

_ahah_render.php

// Lastly, here's a help function pulled from Nick Lewis's blog to alter the form
// see: http://www.nicklewis.org/node/967
// Nested fields are implemented by denmarkin.

// Usage example: $output .= _ahah_render($form, array('block_settings', 'module_block_'.$delta, 'custom_fields'));

function _ahah_render($fields, $name) {
  $form_state = array('submitted' => FALSE);
  $form_build_id = $_POST['form_build_id'];
  // Add the new element to the stored form. Without adding the element to the
  // form, Drupal is not aware of this new elements existence and will not
  // process it. We retreive the cached form, add the element, and resave.
  $form = form_get_cache($form_build_id, $form_state);
  if (!is_array($name)) {
    $form[$name] = $fields;    
  } else {
    // map nested fields
    $nestedFormPath = 'form';
    for ($i = 0; $i < count($name); $i++) {
      $target_el = $name[$i];
      $nestedFormPath .= "['$target_el']";
      // $form = $form[$target_el];
      if (isset($fields[$target_el])) {
        $fields = $fields[$target_el];
      }
    }
    eval("\$$nestedFormPath = \$fields;");
  }
  // print_r($form);die;
  form_set_cache($form_build_id, $form, $form_state);
  $form += array(
    '#post' => $_POST,
    '#programmed' => FALSE,
  );
  // Rebuild the form.
  $form = form_builder($_POST['form_id'], $form, $form_state);

  // Render the new output.
  if (!is_array($name)) {
    $new_form = $form[$name];
  } else {
    eval("\$new_form = \$$nestedFormPath;");
  }
  return drupal_render($new_form); 
}