I think what he's after is for the HUD to look like it's a separate screen to the game screen, without anything on the game screen able to go behind the HUD, so it looks like the game screen is the only part scrolling.
If that's what you're after, then I'd avoid the layout in layout approach too, and us the "top layer for HUD" approach as mentioned above.
Then add dead space to the bottom of your game screen, so that the action can't scroll down to that area, and when you've scrolled down to the bottom of your layout, it will look like the visible area that your characters have reached are the end of the layout.
So, if you have a HUD that 100 pixels high, make sure you have nothing for the bottom 100 pixels of your layout, and make sure that your moving objects can't move onto it.
Hope I explained that well enough.