Sorry for the delay in getting back. Very busy lately.
So just because I have a high contrast preview that looks like a good mask, it's not necessarily. It could just be altitude information? But then I can convert that to a grayscale "bitmap" and use THAT as the mask?
But if I use a distribution shader to confine to a particular band in the terrain, why cannot that be used to tell the displacement shader to only let displacement info through that lies in that band, and everything else is either black or pure white? Since every point in the displacement "map" signifies precise points above the initially flat terrain surface. Ergo, somewhere in the pipeline, those scalar displacement values are keyed to points of altitude. No?