I added a third layer of color negative to the filter, and this makes the sandwich completely opaque to the naked eye. The sun is barely visible (but not at all safe to look at) through it. I can get a reasonable if slightly underexposed image of open shaded areas with an exposure of 4 seconds at f/5.6 at ASA 400. With the third layer there is no difference between the red and blue filters, so I believe whatever I am getting is residual IR leakage.
That isn't snow, it's grass.
A clue to what wavelengths we are seeing is the fact that the streetlight happened to be on, and we can see it, and HPS lights emit a very intense 819.3nm and 819.5nm doublet.