Didn't Walli come up with something like that over at NWDA? I seem to remember Frank showing something like that off in the image gallery.
Have you also thought about using an image texture instead, with a little displacment? I used one in my Gloucestishire image a while back, which seemed to work. You could even use two different tiles of texture, then use a PF to dictate which is shown in the white, and what in the black, to break up any tiling.