Hi Steve,
That is correct, the way to do this is (without setting IAP up in the app itself) is to move the payment method to your site and have the app as a free install and allow them to login after suing your own pay wall.
I can confirm this will pass review. I follow a lot of tech news and watched the problems that Spotify/Netflix had trying to get around IAP and Apple's 30% cut. After they removed alternative payment methods from their app (the one that was in review and kept getting rejected) and reduced the payment method to their site, Apple passed the app for review.
If you don't have Spotify, download it and check out their site and pay attention to their method as great example. Apple and Spotify have a frosty relationship - so anything Spotify currently do and get accepted - you will be just fine too.
I hope it helps and trust that you will pay for provider with the best advice given :)
Best,
El