One obvious problem with that system; what happens with habitually bad posters? Let's say I write something so insipid and worthless that it's worth every downvote on the site... and then a better-quality poster writes an excellent point-by-point take-down of it and gets tons of upvotes for it. Should I then benefit from "generating" such a high quality rebuttal, or is that just going to weaken the already weak incentive structure the karma system is supposed to be creating? I can think of a good case just in the last few days of a poor-quality poster who would seriously benefit from this system, and as a long time poster here you can probably think of more.

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).