I believe it’s actually possible to sign in with mastodon on Pixelfed. Wouldn’t that work for a single user mastodon instance too?

You’re right. I’m not sure if it works with single-user instances, but I believe it does. This is the sort of thing that is technically possible — I believe ActivityPods aims to do something about it, too — but I don’t know… I guess federation can be a bit of a safeguard for this, like having a list of flagged instances that don’t allow account creation; requiring certain thresholds of account age or activity to be passed; stuff like that. There’s also the fact that, being social media, no instance wants bots to run wild, so that could, itself, be a check on that sort of thing, and it might not be economically viable to just host an instance strictly for bot-login purposes, so that is just an inherent barrier to wrongdoing.

Meh, maybe it’s more feasible than not.