Finally, you need to enable the new custom filter by invoking editorContext.getEnabledSearchFilterIds().push('...'), for example, in the init method of a corresponding Studio plugin.

public class FooStudioPluginBase extends StudioPlugin {

  override
  public function init(editorContext:IEditorContext):void {
    super.init(editorContext);

    <!-- 'fooFilter' is the itemId of the added filter panel -->
    editorContext.getEnabledSearchFilterIds().push('fooFilter');
  }
}