Why do all jobs have to be posted at one time once a month?  There is already a "JOBS" tab at the top of HN.  Why not just let other companies post to that page.  You can still emphasize YC companies, stick them at the top for 30 days, and then everybody else below it.  For that matter if PG ever wants to monetize HN adding paid job ads might be the best/easiest way to do it.
Ask HN: Should we agree that this account will submit all Who is Hiring posts?