I would advocate adding the ability to ignore users who join after a specified date. This would allow everyone to freeze the community just the way they like it. I think it would be an elegent hack in lieu of competent recommendation.
I think the trick to preserve the "old reddit" on a new site would be to allow people to choose whose stuff they get to see, i.e. filter by users. That way the early elite doesn't have to get washed out.