Sorry -- I should have been more clear. It's offered in the US (at a significant premium to European pricing), and I actually ordered/received the part. That said, here's the
link on BMW USA's website. Not exactly very helpful!
When I went to try and order the part the first time, it comes up only within the context of the entire floor rail system, so it isn't immediately obvious that it can be ordered a la carte. It was only when I asked for the exact PN that they figured out what I wanted.
It is, however, an extremely well-designed and thoughtful addition to the cargo area if you're often hauling smaller objects in back.