Is the maximum zero fuel weight in aircraft calculated as the forces acting at the max design load limit?
Most can carry centre tank fuel & some stab tank fuel, which must be considered because it's part of the wing / fuselage loading.
I would be sure entirely illegal
- Oh yes!
then what are the actual mechanical / structural implications?
The structure would survive in your example, but you will have no idea of the affect on the fatigue life unless you measured & recorded "delta g" constantly and redo the design maths.