Last night user Ryan asked on Discord how to make a mask based on normal direction. I had some ideas and Allegorithmic's Froyok added another one (number 3):
1) You feed a height map into both inputs of an emboss node, set direction and intensity and then subtract the original height map from the embossed result.
2) You turn your height map into a normal map, use RGBA split node to split the channels and then use a histogram select on either the red or green channel outputs.
3) Again you turn the height map into a normal map and then use a light node.
4) This method fits into the theme of my last blog post. You use the directional bevel node from Substance Share and subtract the original height map from the result. As you can see with some level adjustment this method could work super well for a thick snow layer or something along those lines.
Allegorithmic's NiclasW also commented that it's possible to use a pixel processor. If you know how or know other methods to create directional masks please feel free to add your knowledge in the comments.