That’s not even going to be practically possible for people with no account. You could do it for the instance they’re currently browsing from, but the cookie wouldn’t carry to others if they found themselves browsing from another instance as the sort of “front end”. Might be wrong, but that’s how I understand cookies to work.

Then there’s edge cases like them trying to psuedo-subscribe to a community that hasn’t been pulled down to the instance they’re on yet. If you wanted it to work like it does for a real user, that would have to be logged by the instance so it could fetch it. That would be open to a lot of abuse I would think. Unauthenticated visitors could force federation of illegal comms, or effectively anonymously DDOS the instance by overloading it with requests to pull down tons of comms. There’s plenty of ways to conbat that, but it would undeniably break the concepts behind how federation is meant to work (for the sake of storage and server efficiency) if you allowed it to be kicked off by guest users.

So at best it would be a brittle thing, locked to the instance the guest was browsing at the time and restricted to comms already federated with the instance.

Not completely useless, but it would be a hell of a lot of work for so little benefit.