I like the sound of this, just unsure how this would be able to authenticate an account on behalf of a home instance that's down, in a trustworthy way.

I'm not familiar with the inner workings of Lemmy and the Fediverse, so the following is based on similar implementations I'm familiar with...

SSO implementations usually require the website the user originally registered on (home instance) to confirm the account is real and authenticate it, and in most cases a new user account is automatically created using the SSO authentication details (this would prevent the user from appearing as if they're using their home instance).

To achieve what you want, I think we'd need some kind of way to export the user account and any signing keys used to prove the user is who they claim to be in the fediverse, and then re-import those to another instance. I'm not too sure if SSO would be able to achieve it if the home instance is down.

On the flip side, I'm pretty sure SSO with a Lemmy instance that is active could work. While it would bring a lot of benefit to less tech-savvy users, and a lot of convenience to us when we're given a threadiverse link to another instance, from a technical perspective I think that would be a challenging implementation. Users would need to be careful about having their credentials phished on a malicious instance too