Waitlist for Business Saver with united (you wil have to call them).
Austrian can only sell you PE economy at checkin / airport (if available). Till then United is the ticket owner and they can’t do Premium Economy bookings (might change end of year when United introduces a real premium economy).
Second idea: Try one way booking PE with Austrian (via a travel agency). Online one way prices will be crazy. If that works cancel the United Award ticket.