This, and many of the suggestions in the original thread, and pg's default response, are all falling into the same trap: they are attempting to fix a people problem using technical solutions. Technology can make community management easier. Better algorithms and visual design and so on can improve the way that a community polices itself. But, I think eventually you'll run into vanishing returns from that kind of stuf…
1) Don't you think Wikipedia administrators and Stackoverflow mods fulfil that "gardener" role? (I think so).
2) Consider that above "gardeners" are often criticized harshly on HN (even though, in my opinion, they are a positive force on Wikipedia and Stackoverflow). Don't you think strong moderators would have a bad time on HN?