An inspiration just hit me today.
In the past, I used to play with linear polarizers - when you hold them at 90 degrees to each other, the light gets totally blocked (ie totally opaque). By adjusting them at varying angles to each other, you can get adjust to get a range of light transmissions.
Would the same work for circular polarizers? I was thinking if that is possible, we could actually get a zoomable ND, from ND2 to ND8 to ND100 with no problems.