I suppose you'd need a user interface for authorizing things like that. Ideally, it'd be on the lock screen; show a message which says "New keyboard detected; s

I suppose you'd need a user interface for authorizing things like that. Ideally, it'd be on the lock screen; show a message which says "New keyboard detected; sign in with that keyboard or click here and sign in with your existing keyboard to enable it". It's extra work, and makes it cross-cut through more modules, but it's hardly insurmountable.