Wing root loading a function of MZFW, not wing fuel
For a typical tube with wings airplane configuration, the wings generate the lift necessary to (1) lift themselves plus the fuel they contain (2) lift the fuselage to which they are attached. Obviously total wing lift must match / exceed total weight in order to get off the ground and stay aloft. The amount of load that the wing root must carry is related to the weight of the fuselage and its contents. The the wing root structure is not loaded by fuel in the wing, but is loaded by anything (fuel, cargo, SLF, etc.) located in the fuselage.
It has always been a curiosity to me that for some models, center tank fuel must be counted as cargo weight when the wing tanks are not full, but is not considered for MZFW when the wing tanks are full. From the physics this makes no sense to me - weight in the fuselage is still weight that must be carried by the wing root regardless. For older models with most of their tank capacity in the wings this is not such an issue. For new designs with thinner wings the percentage of fuel in the center tank is much higher.
When we get to flying wing designs, MZFW will be much less of a concern and may actually not be a consideration at all provided MTOW is not exceeded and the fuel on board supports the mission duration plus reserves.