I think it may be possible to do something like what NIP-55 describes with nostrsigner:// deep link URLs which would in theory allow us to switch between native Nostr iOS app / Nostr web app to signer app, and then back to the Nostr client using callbacks. It might be janky and disorienting with all the app switches, but that’s the most seamless experience I can see.

The alternative is the approach I’m taking now in my design, which is using action extensions to bring up a sheet to sign an event without needing to switch apps, but this requires one too many taps from being ideal.