Could you expand on why being an anonymous proxy would be an issue in this case? I can't think of anything interesting off the top of my head.
You can't post (because you're not logged in), so there's no issues with moderation. The toot is already federated publically, so there's no issues with unintentional read access. It doesn't need to contact the original server, so there shouldn't be any load/DDoS issues. I must be missing something…