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 really think only Apple can tell but it is a good chance that it is okay.
I sometimes just went ahead and called them and often I was lucky and had a real competent guy on the phone telling me answers to stuff like that right away sometimes I had someone more cautious but pointing me into the right direction within the legal contracts. And whatever they said at the end the very much shielded app approval team (no phone numbers, no e-mail addresses) will decide. The worst that can happen is that you have to resubmit your app without it. Costs you time but no more.
I always used this link for phone numbers I think they still work.