I wonder how bad it's leaking and from where. I'd want to lay eyes on it and see if I could figure out where it's leaking from. I know McLarens procedure is to replace the trans but I would think there's a grassroots solution in some new seals, rtv, or something.
Did he mention what he was looking to get for the car when he turned down your offer?
Edit: Look at post #74 in this thread.. This guy fixed a leak with an Allen plug and a tap.