Various mailing lists I've run have done well with "if you're not somebody technical or somebody with the authority to hire for this post, hush"

I think it's perfectly reasonable--after all, it's the HN community posting to/for the HN community. That level of personal interaction is really what adds value here.

I'm not a fan of thread-splitting but that'd be one option (Ask HN: Who's hiring directly). Otherwise what about people add tags like [Direct] or [Hiring Manager] to their posts:?

This would certainly cut down a bit on the volume of posts, but I would be happy to see this rule added; I think it would improve the quality of the thread significantly.

I don't think it makes sense. I am the hiring manager, but I only ~5 positions available where I am the hiring manager. My company has ~50 positions open for engineering.

Do you prefer to see 10 posts from my company, from each hiring manager with open positions?

Actually I think I might if only because your team (and manager!) probably affects your well being much more than what company it is.

But I can also see why it seems inconvenient to have many people from the same company post as well.

Ah, this is a good point too; I hadn't considered that. The rule could be "hiring managers only", but as you said, that may be too strict.