I really don't have any suggestions on how to approach that, but I doubt that Playmaker is something you would use to build the foundation of such an editor, perhaps if there were a kit of some kit to make it simpler then you could stack some operability on top of it with PM but this is kind of advanced stuff, especially the way the SC2 editor works with finding nearby cliffs, blending models together likely from a huge repository with lots of rules, choosing random cliff appearances/varieties, etc...
You might be able to find some devblog or post mortems on the topic, but indie RTS games that do that sort of thing are kind of rare. You might find some high level ideas on more generic boards like Gamedev or Gamasutra.