[dms]

I know from a number of reliable sources (e.g., Blaker, "Field Photography") that it works--w/ possible exception at almost complete extinction of light there is a color shift (should be well beyond your application). And you have 3 additional (potential flare) air-glass surfaces so they need to be very clean and not use it where significant flare may be set up.

Actually I do the same thing in photgraphing paintings--I have a linear polarizing filter (PL) on the lights and one on the lens, and the areas that reflect polarized light still polarized appear (photograph w/ film and digital) normal--except darker as one rotates the PL on the lens. Those reflecting surfaces are seeing the effect of one PL rotating over the second.

[rrstuff]

I bought a multicoated haida 1000x Pro II. B+W was tempting but Haida was getting surprisingly good reviews at half a price. I will post some samples when I get it.

[Hornet]

Perhaps it was already mentioned but you could use the multiple exposure feature with exposure averaging turned on. This is up to 9 shots stacked in camera. I have had pretty good results with this - just make sure your tripod is rock steady and you use a remote shutter release.