But the interesting thing is that a 'companion' ticket is in fact subject to the same rules as reward tickets ; so I'm not convinced that ukdoctor's experience can be taken as a reliable guide to what might happen to others in the same situation.

I don't doubt that it worked in his case, because he has told us so. But it seems that EK were not following their own rules. This, from the T&C's applicable to the companion ticket offer (clause no 7 re MBNA/Emirates Elite card) :

".........The card member must purchase an Emirates Skywards Flex Fare in Economy or Business Class at the same time when availing this offer. The Emirates Skywards Companion Reward must be booked on the same flight itinerary, on the same flights and cabin class. The Emirates Skywards Companion Reward will be booked as a Business Class or Economy Saver Reward. Reward upgrades are not permitted".