works in Drupal 8.1.7 only with nodes created by custom module/plugin, not for drupal content types like Article or Basic Page
working:
class CustomController extends ControllerBase implements ContainerInjectionInterface
public function getBlockContent() {
if ($route = $request->attributes->get(\Symfony\Cmf\Component\Routing\RouteObjectInterface::ROUTE_OBJECT)) {
$route->setDefault('_title', $v);
}
}
}