Back in the day most forklift manufacturers offered a "dual" capacity rating on their trucks. For an example the capacity plate would read 3000# capacity at 24" load center to 156" and under that it would read 2700# capacity at 24" load center @ 192". It seems that most import trucks changed the way that their trucks were rated and everyone else followed suit. Almost all trucks today are rated using the weakest link system. Today the capacity plate would read 2,700# capacity at 24" LC @ 192".
In some cases the deration factor is quite substantial. Take a 5000# pneumatic with a 189" triple and side shifter. Although the truck has a "base capacity" of 5,000#, the capacity plate could read 3,400# capacity @ 24" LC @ 189" lift with side shifter. In this case, the truck derates due to 4 factors; the weight of the triple mast, the lift height, the weight and lost load of the side shifter and most significantly the pneumatic tires. The absolute weakest link in this scenario is the tires because if the truck were equipped with either solid pneumatic or dual drive the net capacity would be more like 4,600#.
Forklift truck deration is the most misunderstood aspect of a forklift truck. Most people think as you do, that is that the capacity shown on the capacity plate is only all the way to the top when, in fact it is the maximum capacity of the truck at ANY lift height, even 1". The only manufacturer that I know of that still will supply a "dual" capacity rating is Linde.
I suggest that you go back to your customer and tell him that to lift more than the capacity plate states is unsafe-at any height.