Hi Wouter
Nick is out on a limb with his square top sail anyway so the class rules are not that important in this case.
What you said about it being easy to oversheet a square top sail seems to contradict Gary's experience with his Ashby main (square compared to his previous Irwin sail). He thought he was using an enormous amount of mainsheet tension, but Glenn told him he wasn't using enough. He had to find new ways to get more tension on his mainsheet - he may have even added some purchase (Gary?).
It sounds to me like it's very hard to oversheet a square top main (or even sheet it enough sometimes!).