The HMVC approach works just like a normal request except that it has its own instance of the request class. From the HMVC sub request you can access the parent request object by loading the initial request.
$parent_request_query = $this->request->initial()->query();
You can also access the current request.
$current_request_query = $this->request->current()->query();
You could also just pass parameters.
$params = array(
'passing' => 'data'
);
echo Request::factory('controller/action')->query($params)->execute()->body()