The push notifications are read/intercepted unless you provide a custom push notification service locally on your phone. Can't you do sharing through a specific identifier between apps? 

 You can just encrypt the push message. The Push provider cannot see anything. We do that for Amethyst using Google's and Unified Push's API. No one sees anything.  

 Sure. I'm glad it's considered; that's why I commented. 👍 

 I setup ntfy a few months back and I can't remember what it's for 

 Push notifications of the Foss edition of Amethyst.