How about just arbitrary partitions? Create random sub-communities of 500 active users, show stores submitted between those 500 first, then those top stories to the rest of hacker news. That kills three birds with one stone: You get smaller community thinking, people are more willing to submit stories to smaller groups, and people are more willing to see new stories.
There's tones of ways you can subdivide a community! I am envious of their opportunity to try something. Hacker News is free, they can do what they want! They should have some fun with it.