@Aethelflaed You could try writing out your own evaluation and putting it in the professor’s mailbox. You might even be able to get an official evaluation form by asking the relevant department’s secretary for it, though that seems less likely given your school’s attitude about evaluations. If you think someone other than the professor should read your comments, leave a copy in their mailbox as well.
It might not do any good at all, of course, but your comments might be taken seriously so long as they’re written with a “just thought this could help” tone. I think it would be perfectly appropriate to do this anonymously, though the professor would probably be able to guess that it was written by someone who had already dropped the class. I’m not sure how much you care about the possibility of being identified.