I understood that Apple required that the app and service offer the same subscription plans, so you could not force a consumer to pay a premium (even to cover the 30% fee to Apple). I suppose one could create both plans and prominently feature the iOS one in the app, and the regular plan on the website.

Don't think you need the same plans, just need to make sure that whatever plan is made available through Apple is the same rate as the plan made available through any other channel and that any plan that grants iOS access is made available through Apple. Doesn't mean you can't charge extra for iOS access or have a set of plans that has nothing to do with iOS.