I’m very much in support of this, but I’d want to see some way of preventing people from logging in to the same account over Tor and the clearnet. Or at least some very clear communication on why that’s a bad idea.

Yes, it is possible, as per your linked resource, however it would be up to the server maintainers if they feel it is reasonable to do. Note that federation is not supported over Tor, so federation would go over the clearnet via the original instance. This is not an issue, because the only thing that would be useful for Tor is hiding the client connection IP address, which would be handled under this configuration.

Would there be any issue with abuse? Not saying we shouldn’t do this, I’m for it, but I wonder what downsides we would need to mitigate - I assume trolls and bigots could use this to evade bans?

Maybe users that login with Tor would require special approval for their account before being able to post?

I just don’t want to add significantly to moderation burden.