The difference between NIP-46 and how amber sign event is NIP-46 need signer to be 'online' while amber implementation is 'offline' signer right? I think based on that you can probably write new NIP.
> amber is android signer
I think this can be generalized. iOS probably can do similar things like amber uses android intents. Maybe there is similar mechanism. Probably need someone to make amber for iOS first though 😅