How about requiring end-to-end encryption? Would that not be a better solution?
https://support.pushover.net/i46-are-messages-notifications-...
My understanding is that Pushover encrypts the message with your device's public key (from the app) and delivers it through Apple/Google. On iOS you need a special permission from Apple to "filter" push notifications and this is the hook they use to capture and decrypt it before displaying it on your phone