my personal expierences has lead me to believe that the engine doesnt aid in braking with the described method. Reason being yeah its harder to lock the wheels up and you get more control when the car is in gear however locking the wheels up (test of caliper braking effeciency) with the car in gear and decelerating now try the same stop with the car out of gear and giving the same amount of braking input. The reason its harder for the brakes to lock up when in gear is because the motor running turns hte tranny and axles in a forward motion therefore pushing the car forward.