IIRC every Mark 1 sleeper compartment was fitted with an integrated "gozunder" below the handbasin
They were indeed, although I suspect that many passengers had no idea that it was there especially in their last few years of service. I believe that most of the Mark 1s' forerunners had that too, as did many older sleeping cars in mainland Europe.
Unfortunately that facility was omitted from the Mark 3s, presumably because it was deemed unhygienic by the 1980s. If the Mark 3s had had it, they would either still have needed the troughs (thus defeating part of the object of having retention tank toilets, which was to eliminate the need for the troughs) or each compartment washbasin would also have needed a retention tank or some kind of water filtering system.