Hey PG, have you read Clay Shirky's Here Comes Everybody? It has lots in there about the problems inherent in big communities, especially in how they function on the internet.
The later parts of the book (chapter 8 and beyond) seem applicable to the problems HN is facing.
PM me if you want, I'll send you my copy of the book.
I haven´t read that book, looks interesting.
On the topic of moderation I would suggest the ability to somewhat "follow" good commenters, and even perhaps block bad submitters. This way, reputation is not only karma, but quality of followers.
Please get in contact with Matt Haughey, he's the best possible person to talk to about this, having run Metafilter for over a decade.
Be warned, he's going to strongly advise you to set up a backchannel for public metadiscussion. Jeff Atwood resisted it vehemently at first when another metafilter moderator was a guest on his podcast, but they did eventually set up their meta subsite in the same vein.
I'd actually been considering hiring someone to run HN, though not to moderate it in quite as hands-on a way as you're suggesting. Interesting idea though.