I have a hole in my knowledge of Panels that needs filling :) I created a DS panel for the node display for Commerce product-display. Works fine.
Then the requirement changed a bit, so I decided that the product/xxx URL will be controlled by Panels, using a 2-column panel, and that the left side will be the DS layout.
There is a DS choice in Panels when adding content, but it is not for selecting a DS layout.