Would a spammer really go through that just to submit a spam link that probably wouldn't get beyond the new page anyway? It's quite a bit of work, even if you have the same framework that you can use across multiple sites. My experience is that a post that doesn't get upvoted will receive around 30 hits.
I don't think it's feasible to make "write once run anywhere" code for this. Input fields will be named differently, rules will vary and spam filters will trigger on different things. Even if you only have to spend an hour grooming your script for HN it seems like a lot of work to get 30 hits. You can of course reuse the script on the site once you've made it, but it won't take much before PG figures it out and bans IP-addresses, or some other measure.
Heuristics aren't terribly hard. Scan for an input text box of a minimum certain size. Then try each submit button independently to see which one gets your sample text on the page.
Surely there are easier ways of getting 30 hits.