I've never tried this, but theoretically this may work:
In cruise, with the power set sufficiently, you could hold the autofeather switch in TEST. The system should arm, and then you would simply perform the test as you would on the ground. I would be extremely cautious if you had to lift the power lever though to get the torque below 350 due to the probability that the prop blades would go into ground fine, which would cause obvious problems.
In the end, I agree with the previous poster, probably best to write it up and let maintenance figure it out.