When I used to fly the A320, I spoke to a pilot at Airbus about loading. The company I used to work for at the time were trying to 'invent' their own rules, so we kind of fell out over it and I left. The Airbus Guy was really helpful over the matter. Basically, no-one knows the aircraft better than those who made it, not just because they made it but because they get feedback from all the operators. The operator can propose what they like, but to become an operating proceedure contrary to that of the manufacturer, then they would ask the manufacturer if they can do it. If the manufacturer agrees then an ammendment to the Ops manual will be drawn up and sent to the Authority for approval. If the Authority has no objection then they rubber stamp it and we then comply with the requirements of the manual. So in answer to your question, it is most likely that the manufacturer is responsible for the requirements to uphold, but in any case wherever it comes from they would have been involved at some time. Even if the Authority was the first port of call, it is doubtfull they would approve an amendment that effects the aircraft loading without first contacting the manufacturer.