Keep in mind that you have virtually infinite depth of field with a pinhole, so any dust speck on the filter is likely to show up somehow.
You might try putting a gel filter inside the camera.
Though, with effective ISO of 6, and reciprocity failure, you might be in for some very long exposures, even at mid day. I would bracket exposures, for sure.
"Often moments come looking for us". - Robert Frank
"Make good art!" - Neil Gaiman
"...the heart and mind are the true lens of the camera". - Yousuf Karsh
never had that kind of DOFand my dust is always in the sky for reasons of gravity. no bracketing;just f/256 andbox speed and normal reciprocity compensation but often to make paper negativesinstead of film