No.
But if you think about it, having the brake boss to the rear means that a lower torque on the fixings can be used (an important issue with carbon legs). If the brake is mounted on the front, the rotation of the wheel combined with the leverage of the brake mount tries to push the boss away from the rim when the brake is applied reducing it's effect. If the brake is rear mounted the opposite is the case, the boss is pulled toward the rim and brake force is maintained without the need for overly tight mountings or a booster of some kind.