From what I can remember the web app can open Amber and send the event json but in amber I can't send the event or signature back to the web app. Every web app would have to create a dialog like nostr.rocks does and in amber you just copy the signature back to the web app. I have to test this more to see if I find a better solution.
Best solution right now would be to implement this in nostrnet.work apk or Spring 

 Apps like Amber (NIP-46) should also just handle the NIP-7 stuff. And apps like Nostore (NIP-7) should also handle the NIP-46 stuff. cc @ursuscamp