John,
I had a lot to do with Airbus loads and stressing,and I have never heard of any such negotiated reduction.
The big difference between now and then is that where in the old days resources limited one to studying say half a dozen cases on whatever depth ones budget and skill allowed, nowadays it is common to calculate the detailed loads for many more cases and many more locations and then to scan the total data set to get the worst case at each critical location. It is a different world from adding a bit of strength in case.
As a matter of fact, quite a lot of the airframe these days (or at least in the last days of metallic construction) is/was designed not by static strength but by damage tolerance, so there is a margin over the static strength requirements which I think was not there at the time of Zaphod's 727 incident.
Zaphod
Difficult to say how an A320 would have come out of that because the "Gs" are not known. A spiral dive of itself doesn't generate much g so it would depend on the severity of the pull up. All aircraft are designed to withstand 3.75g ultimate load at MTOW, so at lower weights they can stand much more. That 727 I think was at the end of the flight so it might well be capable of (say) 5g. In similar circumstances I would think that about right for an A320 also.
The overload due to high speed gear extension is another matter which would depend not only on the overspeed but also the detailed design of the gear.