Applying fractal noise to an existing heightmap doesn't always quite seem to work right with areas of the map selected (unless I'm not using it right), hence the workaround in the example script of saving a noise heightmap to file and then applying that as an image later (though that also allies me to overlay multiple fractal noise maps at different scales). But when I referred to "noising up" the map in Gimp, I meant more in the sense of applying lots of little peaks, ridgelines, and troughs with the dodge/burn tool; the sort of thing you don't want to be truly random but would be tedious to do in the initial sketch with discrete levels. I also like the "pick" noise option there, though I only use a very light pass of it so I'm not sure it actually has any impact on the final product.