The flight instructor and pilot-rated student were practicing off-airport landings and takeoffs in a tailwheel-equipped airplane. After landing on a gravel bar, the instructor and student surveyed the takeoff area on foot. During the subsequent takeoff run the airplane was not accelerating as hoped, and just prior to the abort point, the instructor told the student to abort the takeoff. The instructor joined the student pilot on the controls. During the attempt to stop the airplane the brakes were applied too heavily, and the airplane nosed over. The airplane received substantial damage to the rudder and vertical stabilizer.