I don't know much about HMPs but I managed to do a test. It's not a precise method, but it seems to work. HMPs seem to act very strange in GS. It's very different from MDLs.
Since everyone's project is different, you'll have to use your scripting skills to adapt it to your level. You'll see what I mean when you run it.
This test is very ugly, because I simplified everything so that you can see how the scripts work together.
DOWNLOAD TESTHope you have a nice day
EDIT: I forgot to delete this line from the terrain action:
my.polygon = on;
Delete it in your wdl, because it's only needed on MDLs.