App Store Review Guidelines Clarification

My thoughts: you'd be violating the guidelines, but your app could get approved, of course. The payment does indeed not occur inside the app, but the application "utilizes" such a system (which is very broad) thus is in violation.

This reminds me of the Newsstand/subscriptions discussions going on before. Basically, if you offer something outside an app, you have to make the same (or better) offer inside the app (via IAP subscriptions). Perhaps this is applicable in your case, too. Although, according to 11.3, you may not offer services outside your app if purchased via IAP (so you may not unlock features on e.g. a website too.)

You'd also try and offer a free app. Once users (somewhere, somehow) upgrade their account, they can access the members only app, a new, separate app. But approval is still questionable, which brings me to my last part:

"We will reject Apps for any content or behavior that we believe is over the line. What line, you ask? Well, as a Supreme Court Justice once said, "I'll know it when I see it". And we think that you will also know it when you cross it." — https://developer.apple.com/app-store/review/guidelines/

In short: submit, pray and find out.