The nice thing is that the comments are all public so if someone wants to take a crack at building a state-of-the-art sentiment detector or what have you, they can have a go—and if anyone comes up with anything serious, we'd certainly like to see it. As would the entire community I'm sure!
You can probably put a big dent in the number of low-quality comments by just showing a “hey, are you really sure you want to post this?” confirmation prompt and display the site guidelines when you detect a low-quality comment. That way you can have a much more relaxed threshold and stop worrying about false positives. Sure, some people will ignore the gentle reminder, but then you can be more decisive with flags and followup behaviour because anything low quality that has been posted will by definition already have had one warning.
I don't think a confirmation prompt will help because people tune such things out after they've seen them a few times.
You get better intent assessment than with NLP/ regex/whatever.
Plus HN is entirely in English, so you never have to worry about lexical resource gaps.
There is no off the shelf solution - afaik. In addition I have no idea how expensive running costs will be.
But something serviceable can be built.
Source: mod /t&s person dealing with these things