HN is (roughly) open source:
https://github.com/nex3/arc/blob/master/news.arc although it doesn't necessarily correspond to the currently running version. You can do the facelift yourself via any number of Stylish userscripts.
Reddit's source code is going to be relatively specialized for their architecture (Cassandra and EC2 and whatever else), and piggybacking on a subreddit is going to encourage bleed, lower control (e.g. pg restricting signups at times), and change the value of karma to represent a cross-culture value instead of what's valued locally.
What advantages are there for merging? You save one password?
