Ask HN: SaaS apps in the iTunes app store how do you market an upgrade in app?

We are a SaaS startup with an app distributed through the iTunes app store. We aren't using Apple's subscription billing, so we are restricted by their rules of not linking back to the main site where subscription and pricing information may be found.

We're putting together our free version now, which disables some functionality in the app. I'd like to leave these sections in the app but replace their functionality with marketing pages to ask the user to upgrade. I'm sure though, that this would be against ToS.

Does anyone have any experience with how to properly word this kind of message and stay within App's ToS?