If you want to make in-app purchases on iOS through users stored Apple Store payment details you have to follow Apple's terms - there is no way round it.
Even if you have a separate "subscription" to your application/game on your website, you cannot promote it within your application, it will never get approved.
What you can do is:
- Offer "free" time-limited trials within the application in exchange for the users email/facebook details
- Send them marketing email/facebook messages to subscribe on your website
- Add code to your application to check your server for "is paid/subscriber" when the app starts
Good luck!