My tiller cross bar broke at the same spot a while back, luckily Matt had another! I'm seriously thinking of replacing the whole mess with the Ronstan A cat system, carbon crossbar with the clip on tiller extension. I had this on my Bimare Jav 2 (fiberglass, not carbon) and the only problem was you had to tape the cip on part from the tiller extension into the clip on the cross bar, because it was prone to popping out.

I don't think the breakage is only from doing the back over it thing as you described above, although that doesn't help, but every time I jibe I tend to put downward pressure on the crossbar as I steer and cross under the boom. That was when it broke on me, during a jibe. I also lost my tiller extension to the sea, but finished the race by steering with the stump that was left.