I know how to measure a glider's pitch moment of inertia. In our hangar, we use a hanging cradle to lift a glider for more floor space. This means that we essentially have a pendulum with a glider at the bottom.

You could measure the period of oscillation with a glider and without. The extra mass of the glider wouldn't change the period of oscillation, but the moment of inertia would. I've used this technique to measure rotational inertia of bicycle wheels, and I think it's possible to get within about 5% accuracy.

https://en.wikipedia.org/wiki/Moment...pound_pendulum