Kind of ridiculous design BMW did. You only get the passthrough when you order seats that can already fold down!? If your seats fold, you don't really need the passthrough anymore, do you? The non-folding seats aught to be the ones with the passthrough.
My seats fold, and whenever I go skiing I just fold down one seatback. Skibag has never been unfurled (looks like it'd be impossible to get it back in the case if I ever did!)