That was my first thought as well...

My only guess would be if the polarized gel filter was actually a circular polarizer (unusual for a gel filter). Circular polarizers consist of a linear polarizer layer on a 1/4 wave retarder layer that then rotates/circularizes the polarization. The coating on the C302 and other instument glasss is actually such a circular polarizer, with the linear polarizer layer outermost--bouncing off the glass changes the circular orientation handeness, back through he retarder layer again turns that circular polarization to linear but at 90 degrees to the linear polarizer layer, blocking the reflection.

If a circular polarizer film was held with the 1/4 wave layer side facing the pilot and the linear polarized side to the C302 glass and rotated so it is aligned to the linear polarizer layer on the glass then the pilot sees circular polarized light that his polarized sunglasses won't show orientation sensitivity to, but the overall image will be darkened... how is that for a hand waving guess? An obvious test is this will only work with only one side of the polarizer film facing the pilot, turn the film over and it will not work (it will go from dark to light as you rotate the filter but it won't improve problems with viewing with polarized sunglasses).

Darryl