The thing is, I could, but because I'm not at all related to HN in any official way, it would be up to me to change how the API gets data from HN whenever HN changes anything at their end. That would mean there'd probably be a delay between HN making changes and me making changes, during which time any applications using the API wouldn't work.

I'd happily do it with the blessing of the officials at Y Combinator :)