HTML_Quickform2 custom rendering

I've been playing with the callback method of customizing the rendering of
output.

 

I can modify the form rendering with:

 

$r->setCallbackForClass('HTML_QuickForm2', function($renderer, $form) {

.

.

.

});

 

 

 

I can modify the element rendering with:

 

$r->setCallbackForClass('HTML_QuickForm2_Element', function($renderer,
$element) {

.

.

.

});

 

However when I define:

 

$r->setCallbackForClass('HTML_QuickForm2_Node', function($renderer, $node) {

.

.

.

});

 

The default: _renderLabel() function is still run instead of the custom
callback function.

 

Is this a bug?