At the moment I can't think of a terribly easy way to do this in TG2 yet. You can do displacement on a horizontal plane as one idea, but you can't do populations on that anyway so that's out. As you know, you can't populate on objects yet, so that wouldn't work either. It really looks like you're doing ok with Houdini at this point so I have to recommend continuing down that path, though I wish TG2 could meet your current needs.
- Oshyan