For cases like this, when people leave their key permanently attached to a device, it would be nice if the device itself had this functionality built in. I mean, the iPhone already has a secure enclave to store this kind of sensitive data.
There is a web service using exactly such a mechanism for U2F in browsers: https://krypt.co/
This works through an addon in the browser rather than native functionality but the system is secure enough that I use it as a backup for some of my 2FA services in case I lose my TOTP keys.
