Would there be any issue with abuse?

Not more so than the current anti-abuse methods blahaj already uses. A valid e-mail address, a short paragraph explaining why you want to join the server.

Registration can also be limited to actual IP address.

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

Locking until your comment score gets high enough and being 30 days old? Or one post/comment a week for the first month?