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.