For these 2 rules:
11.1 Apps that unlock or enable additional features or functionality with mechanisms other than the App Store will be rejected
11.2 Apps utilizing a system other than the In App Purchase API (IAP) to purchase content, functionality, or services in an app will be rejected.
Is the applicability of these rules reduced/removed if the enablement/disablement of the features/functionality (11.1) or the content purchase (11.2) does not actually occur within the app on the device.
For example, you write an app that requires free registration but if you visit a website outside of the app (and not linked to from the app) to “upgrade” your registration (by paying money) the app gains some more functionality or content next time you use it.
Thoughts?
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.