G4P imagebuttons setZ method protected

I'm using G4P ver4.0 library in processing 3.0.
I want to have labels/imagetogglebuttons displayed over top of other imagetogglebuttons.
I have tried creating the buttons in various order, but I cannot seem to control which one appears "on top".
I found the method setZ in the GAbstractControl, but it is a protected function.

How can I set the Z-order/depth of various GUI elements in G4P ?

Thanks,
Dan

@quark