Apple watch app without functionality I have a client who wants to add Apple Watch functionality to its app. However, my client wants to take paid from its customers using In App Purchase to activate this content. This will lead to an "empty" app on the apple watch with informative text to the user to purchase the Apple Watch IAP package to enable its functionality. 

Although the request from my client is logical, I have a hard time believing Apple will approve such an app. Also, with regards to 2.12 of the review guidelines:


  Apps that are not very useful, unique, are simply web sites bundled as
  Apps, or do not provide any lasting entertainment value may be
  rejected


I can imagine Apple will interpret my client's Watch app as an app that is not very useful, until the user purchase the functionality.

Anybody with any experience here or having more insight on whether it would be worth giving it a try or not? Last thing we'd like to do is waste our time ;)
I would say you are over thinking the watch app.

Watch apps on Watch OS 2 and iOS 9 are extensions. They are the antithesis of a functioning app and simply a little detail or a little tidbit.

Furthermore, Apple approves apps that do absolutely nothing without a paid account. Netflix, Casper Focus, Splunk, all the newsstand subscriptions where you need to pay for content before reading an issue.

Now - Apple's review guidelines let them have great latitude to deny apps for no reason at all - let alone 2.12 "not very useful" which itself is highly subjective.

What you are concerned about also concerns many other app developers and it sure would be nice if there were a way to pay for "Pre-Approval of Innovative Features" so you could invest maybe 5k of effort to prototype and then pay for a review by Apple before investing the actual amount of development work in your app. See this story for the details of how pre-approval and the developers surveyed are in similar situation wondering what might pass review:


https://www.macstories.net/stories/developers-apples-app-review-needs-big-improvements/