Wouter
The quite a few of the HT's went with the double forestays and didnt have a problem gybing the kite. Generally the pole is out in front of the bows and the forestays are attached a ways back on the bows, so I dont see how it will be an issue on an 18ft boat - A cat, HT, etc.

It might be an issue on an F16 because you have less bow, perhaps, but on an 18ft boat you wont have a problem. Keep the pole long and the luff long and you wont have to worry about the forestays interfering with the sheeting angle.

As far as keeping the pole supported, I think you could use sticks and mount them over the forestay eyes and make it removeable. This would support the pole and keep it in place without drilling holes in your bows. The other issue is on a modern A cat, the bows dont come to a point (XJ, A2, etc), so there is no perfect mounting spot, so I would move it back to the forestay eyes.

Bill