javelin,
In the case cited in your latest posting, you need to think about the sequence for burning fuel.
Adding fuel to the wing tanks does relieve the wing root bending moment so more fuel can be added to the body.
I believe you are equating the fuel added to the body being the same as adding cargo.
This isn't valid because, as stated in M(S)F's reply, the airplane still needs to be good for 2.5 g's Flaps Up with no fuel on the airplane.
That's why the first fuel burnt is the body fuel.
As the body fuel burns off, the wing root bending moment reduces.
As the wing fuel burns off, the wing root bending moment increases, with the max. bending moment reached at MZW.
As an example: (values are notional)
Load Condition/ Weight/ Wing Root Bending Moment
MZW/ 61 tonnes/ 661 meter-tonnes
MZW+Wing Fuel/ 73.6 tonnes/ 547 meter-tonnes
MZW+Wing+Body Fuel/ 77.0 tonnes/ 581 meter-tonnes
(It's hard to do a good table)
For this example based loosely on the numbers you provided, the highest bending moment occurs at MZW .
If the gear could stand it and tail loads were O.K., you could argue for a higher gross weight and more body fuel to increase range, but MZW would still be limited to 61 tonnes if the wing root limit was 661 meter-tonnes.
Of course, other loading conditions might be designing MZW, such as fatigue.