Waterfall stacking: Can you leave your tripod and ND filter at home?

How about two shots?  One with the polarizer and the other without.  Load them as two layers and brush out the water (or upper picture) in one of the layers.  You probably do not even need to remove the polarizer, just turn it.  Careful not to move the camera.

I do this all the time with exposure, but no reason you cannot do it with filter too.

--
Phil