The test that you describe (looking for change in resistance with change in voltage) is similar to what is done on rotating equipment.... where you measure current vs time. It's called dc step voltage test and described in IEEE 95. Timing of the measurement during the test is important because there are multiple components of current present during the test: absorptive, capacitive, resistive. The first two vary with time after an increase in voltage, the third is typically the one you're most interested in.
For rotating machines, the current vs voltage tends to be constant with voltage increases (when measurements are timed properly ... typically one minute after each step) for a normal healthy motor in the normal voltage ranges (up to and beyond operating voltage). That corresponds to constant resistance with voltage. If the winding is unhealthy, then the current starts to increase greater than linearly. That corresonds to decreasing resistance with voltage.
Since dry transformer insulation is somewhat similar to rotating machinery insulation, I would think a dry transformer insulation test would act similar to a rotating machine test as described above. For an oil filled transformer, all bets are off.
Of course if you are aiming to test your transformers insulation condition, I would stick to more standard tests. That leads to the question: why do you ask?
=====================================
(2B)+(2B)' ?
