Hey Tom!
I got brave and ambitious and tried it on a test stack. Backgrounds (or groups) layered upon backgrounds, layered upon backgrounds, seem to present no particular problem to RR. That is handy. It's HyperCard alien, that's for sure. It violates the "stack of cards" paradigm. This isn't your father's hyperCard
As a HC retread, I have a tendency to think of one group as "underneath" another one, but that probably isn't quite right, except for the sake of the graphical display. As far as I can tell, a button in a group lying "underneath" another group of buttons receives its mouseClicks normally. It isn't quite like one stack lying "underneath" another stack.
I guess this is why button families have been abandoned. Buttons can be literally grouped any which way, and will behave as if they are in families, and will still get copied to new cards, if "behave as a background" is turned on, just as if there were only one HC-like background.
Thanks again,
Tim