If that ever changes and HN is overrun (insert dramatic and sad Hans Zimmer score) what about starting a new HN just for programmers? I wouldn't be able to get in at the moment, but someday I'd hope to be able to.[1]
[1] I'm assuming you'd have some kind of rotating or individualized Project Euler style problems that you have to solve to get in, and that they would be pretty tough. It occurs to me that you could also have temporary student accounts for people who are just getting into the software world as well though.
Passing a programming test is no guarantee for having anything interesting to contribute to an online community, just look at your company's internal IT mailing lists.
An invitation based system, while not perfect, would probably be the most effective method. Plus, the owner of the site could restrict or encourage growth by the amount of invitations allocated... allowing for easy management of unwanted lulls or spikes in user activity.
Invitation based communities are also very self-policing. If you invite someone and they happen to be an idiot then there is egg on your face as well.
By "get in" do you mean to comment or to simply read the comments? I'm all for a intelligence test (I personally would like a "actually RTFA test"), but I'm pretty sure that making that test required to even read the comments is a terrible idea.
Probably because your random internet stranger has never heard of HN.
It's a shame, for instance, that I was ever allowed to find this place.