I finished my macro “postpiles”, inspired of the discussion in this thread
I used it in a teamwork with Hillrunner, you can see the result (those flat shapes in the ground) here:
http://www.renderosity.com/mod/gallery/index.php?image_id=1249598
And you can download the macro here:
http://terrafgen.free.fr/Downloads/Macros/WM1/Postpiles.dev
It requires the Standard version of WM, because it uses a Blur device.
Enjoy 