personally i think the problem lies in incorrect use of terminology, it is not a cat it is a dpf if people refered to a diesel particulate filter as a dpf and not as a cat i doubt there would be the confusion on the matter.
when it comes to test time i think problems will happen with diesels not having a dpf due to poor mot testers lacking knowledge of the difference between a cat and a dpf but it is quite clearly written in the manual to only check for a cat if it has a full emissions test which a diesel does not iam sure if vosa wanted a diesel failed for not having a dpf it would state to check for one.
which is why i say if your diesel fails for not having a cat challenge the garage and appeal!
knowledge is power!