So I know that there are no promo codes for in-app-purchases in iOS. What I am wondering, is will Apple reject this mechanism:
Provide two in-app purchases, one
I believe your proposed method would work because a) you are not linking to an outside site to purchase the product (i.e., you are still using the App Store iAP method) and b) there are other programs that use similar features (e.g., entering in your OpenFeint username to enable that service).
Good luck!