In a hydrogen fueled plane, could part of the fuel load be allowed to become lighter than air to offset or trim for the load?
The buoyancy of hydrogen is not as high as expected, and is even lower at high altitude due to the lower ratio of the density of hydrogen and air.
Just for comparison:
The tail tank of the A380 has a capacity of 23698 l.
When this tank is filled with kerosene / air / hydrogen, the content weights:
Substance           | Density [kg/l] | Weight of 23698 l [kg]
--------------------+----------------+-- -----------------
Kerosene            | 0.8            | 18958
Air (sealevel)      | 0.00122        | 28.91
Air (10km)          | 0.00041        | 9.72
Hydrogen (sealevel) | 0.00009        | 2.13
Hydrogen (10km)     | 0.00003        | 0.72

At sealevel, the lift of this tank filled with hydrogen is 28.91kg-2.13kg=26.78kg.
And at altitude, it's just 9.72kg-0.72kg=9.00kg
This is nothing compared to the 18958kg of kerosene, and shows that it makes no sense to fill any volume in an aircraft with hydrogen for trim.