I guess Steven asks here as that other printer is quite taciturn...
Possibly you can put 2 polarizers so he can have a variable ND filter.
To darken the sky of course and hold back the exposure for the rest. Use the darkening part of the polarizer on the non sky part and it will get lighter the sky on the other hand about 1/3 stop darker as it receives about that amount more exposure. Of course it could be that I am talking complete rubbish but it kinda seems logical from my point of view.
You could darken the sky if that light is polarized. What it partially is in nature.
On a film image though that polarisation will no longer be there.
The issue discussed here is about printing with a polarizer on the enlarger lens.
You say he was a mural printer? Did he use a mirror under the enlarger to throw the image onto mural? If that's the case, then polarizer would make a lot of sense to eliminate the double image from the mirror. You see, unless you use the first surface mirror under the enlarger, you'll get two reflections - one from surface of the glass and another from the silver surface. It's been a while since I've taken the optics course, but polarizer might eliminate one and not the other as such improving sharpness of the image. However, I'm really just speculating here :)
I found some sources that claim that using polarized light in the enlarger gives deeper blacks and crisper clearer images
Seems like an interesting way to slightly mess up the sharpness and color balance of what you're printing. The only thing I can think of, is that
certain Durst enlargers could also be used as copystands, with a special sheet film holder (LARKA) going into the negative carrier position.
Then the system could be cross-polarized, with polarizing sheets over the lights themselves, and cross-polarization on the lens. But for
printing???? Makes no sense to me. I guess you could turn an enlarger into a giant polarizing macroscope if you had unlimited funds - that would be a fun pictorial project for rock slices thin enough to transmit light, but would probably be useless for true scientific purposes.