That's the only info I have, it's from a former CAA examiner with whom I was discussing the work I was doing and what I wanted to do. I am not certain that the event related to JARs, it may have been under the previous CAA regulations. He said it was definitely illegal to carry out aerobatics as part of a trial lesson, the reason was that they did not form a normal part of the PPL syllabus and that the CAA had either prosecuted or threatened prosecution to stop it happening (I can't remember which he said, but I think it was a prosecution).
My assumption is that there is an underlying presumption that to have a useful lesson in aerobatics the student must be able to fly already. If that is the case you might then justify teaching aeros to a PPL student, but not to someone with no experience.
I entirely agree with you that aeros are a great way to sell aviation, and I would say that it should be allowed as there is no added danger (and I think we are greatly over protective in our regulation of certain areas of life anyway), but I am not the one that needs convincing. You are not selling a marketing device, you are not allowed to. The organisation is only allowed to charge for its pilots to fly someone in a lesson.
The requirements to take a multi-engine rating are to have 70 hours as PIC. I don't have a copy of LASORs to hand, but that is from 3 different approved FTOs, so it seems reliable. Even if no regulation states training cannot be taken before this, then a trip that cannot count towards the rating or licence, and with the student a minimum of 100 hours away from being able to take the rating course, would be very hard to justify as a lesson in a court of law.
Remember you have to justify the trip as a lesson, as a court may well take the view that the CAA does not have to show that the lesson is banned but that you have to show that the trip is valuable as a lesson rather than simply a fun flight.
One thing I learnt from another friend who is an expert witness in aviation cases is not to go simply to the letter of the regulations. You must also always be able to justify your decisions in a court of law if anything goes wrong and you are sued or prosecuted.