I suspect it's not practicable to design an FQ system that reads accurately in all pitch and roll conditions. If the FQ reading is accurate on the ground and in level flight over the normal speed range, that should be sufficient.
The capacitance probes in modern FQ systems inherently compensate for fuel density, so they are as accurate as you can get with current technology.
In the 747 specifically, a high pitch attitude at low FQ is NOT a good situation, since the forward boost pumps will cavitate. There is an FCOM warning regarding go-arounds with FQ <900 Kg in a tank.