I talked to them again and although you arnt supposed to modify a racing boat....
They told me that if I were to use threaded bar ( they winced), and tie the forestay to the floor, the pull would try to rotate the fitting around the anchor in the floor, which would change the loading percentages...
The pull would now be in the direction of the blue arrow...
There would then only be a 20% pull trying to pull my patch out of the deck, and an 80% pull trying to pull up the nose of the boat.
I decided I liked the chances of that a lot more than relying on my fibreglass patch not coming unstuck.
I was still a little worried though....