The idea sounds great, but there is a big caveat. I think it will draw a lot of crowd to HN, which PG wouldn't like. One one hand you need to keep the registrations open for newcomers to ask good questions, on the other hand a lot of signups will be from a section of Redditors not used to HN etiquette. I remember that signups are sometimes disabled on HN whenever there is a popular link to here from Reddit. Also, HN…
I think signups are an interesting point. Nobody wants the site to be diluted too quickly and lose what makes it good, but I do think that a wider userbase would be a good thing for HN to stop it from being as much of an echo chamber as it often is.
A wider userbase only exacerbates the echo chamber, as the most popular, agreeable, easily consumed content rises to the top much faster.