Hopefully this *clarified* Requirement doesn't trash everything we've been working on?!
Ummm...it probably does.
But don't despair. It just means that your business model isn't correct. And you need to adjust it. It doesn't impact the derived vs. natural keys decision. It doesn't impact your idea of having a hidden "dimension" that sits outside the perceived navigation.
Just keep working on it until you are truly happy with the model.