There is new support for dealing with invisible targets, such as empty layouts. First, when you select these (which is done automatically on drop, as well as if you click on these items in the Outline), the layout itself is shown as a 20x20 pixel bordered rectangle.

When you initiate a drag & drop, whether from the palette or from within the canvas itself, all such invisible views are automatically shown (and the layout recomputed if necessary) such that you can target these containers as a drop target.