Turning off embeds in all comments, site-wide, would be easy -- but I don't know if turning off embeds in just guest comments is "an existing feature they can turn off" so much as "a new level of differentiation they would have to code for."

I've used locking/comment-limiting on specific works where it seemed called-for, and I'm glad AO3 has the options! Just sad about any of them becoming site-wide defaults.