I wouldn't be anything to do with the CAA they would claim.
If the FTO wanted to do a flight then a 170A thats what the student would have to do. You would also have problems that if there were any syllabus differences, they would have to be covered as well.
And if it was a new type of twin there would also have to be differences training.
I agree with you though it would be twisting the knife after it had been stuck in.