I’ve just had another, completely stupid but not implausible, idea:

> a local internal WSCD, which is a component within the User device, such as a SIM, e-SIM, or embedded Secure Element,

So you could issue SIM-cards / eSIM profiles that only do signatures and nothing else. The app then connects to such eSIM (and you keep your main SIM/eSIM in another slot).

The less stupid variant is, of course, to get mobile operators to issue SIM cards with e-sign capabilities. Estonia has that, for example: https://www.id.ee/en/mobile-id/