Thinking a bit about it I don't think it would be a good idea to add the generated karma to the top poster or even a fixed fraction of it. The fraction should decrease with distance from the root because all intermediate commentes also deserve a share of the pot and the pot shouldn't increase by itself. The function of distance should be between harmonic and exponential I think. Or it could be tuned by sampling actual comment trees (only that your whole idea is to influence the shape of the tree).
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.