function FilterForm() {

//...

      $actions = new FieldSet(
         new FormAction( "filter", _t('UserDefinedForm.FILTER', 'Filtern') )
      );

//...

return new Form( $this, "FilterForm", $fields, $actions );
}

   function filter($data, $form) {
      $ids = $this->someComplicatedFiltering($data);
      $_GET["ids"] = $ids;
      return Director::direct($this->data()->URLSegment."//select");
}

function select() {
return array("FilteredIds" => $_GET["ids"]);
}

$this->pushCurrent();

$controller = new ModelAsController();

$request = new HTTPRequest("GET", $this->data()->URLSegment.'//preview', $request->getVars(), $request->postVars());
$request->match('$URLSegment//$Action/$ID/$OtherID');

$result = $controller->handleRequest($request);

$this->popCurrent();
return $result->getBody();