On a related topic: I reviewed Georges Tillard's Catamaran Sailing DVD last night and noticed that it recommends easing the main a bit during a gybe and then trimming in again after. I wasn't taught to do this and wonder if others regularly do it or not. (I was taught "Keep the main tight downwind, it's your backstay.")
I never release the main jybing, but it may be due to the fact that I sail mostly solo: not enough hands to handle yet another line! And I don't keep it *that* tight, there is a bit of twist in there.
This said your mast should be safe even if you release the main a bit while you jybe: the spi doesn't pull that hard as it is not sheeted in fully powered and as long as the mast rotation is fully off the diamond should hold the mast together. Plus F18 masts are bulletproof.
Incidentally I noticed in Carnac that both the Cirrus R, some of the top WildCat and the Phantom were equipped with a little line to force the rotation of the mast, no idea when they use it, but I could see it helping to protect your mast in rough conditions. Don't forget to release it when you jybe however
