From the O/P...

Today, they were sent a message by a student union offical. The message, I’m told, is confidential and so can’t be reproduced here

Rot. If someone sends you a letter or email you are at perfect liberty to disclose the contents, providing it doesn't contain commercially or medically sensitive data relating to a third party, matters relating to national security or otherwise breaches the Data Protection Act. Providing the names of complainants are removed there is no reason why the message from the student union cannot be reproduced.

Of course the student union official may feel it strengthens their hand to make such a claim, but it is important to call their bluff on this as well as the cartoon.

If it were true that student union officials were legally entitled to send out correspondence and prevent recipients from disclosing it to others then that would be an even more alarming threat to free speech than attempts to restrict J&M doodles!!