Cool. I'll have to try this. It looks like you might be able to use the alpha-and-color option to cause interesting variation in tree(or mountain or whatever) color. For instance, with trees, create a layer with a narrow variation of green shades. (Possibly use a greyscale fractal and gradient map with a narrow gradient(say dark green to light green)).
Another idea I have is to make an initial color layer that varies only in hue and saturation, and apply a variation in value by multiplying with the shaded relief from an HF.
Then you paint in your desired forest density in alpha.
Might be a good way to add an interesting forest texture to a shaded relief map.
I was initially thinking of using top-down trees for a realistic look, but I think this might work best all symbolical, with iso trees simply as a symbol saying, "forest here."
If nothing else, this is making GIMP look like a more and more viable alternative to CC for "symbol"-type maps.