I've gone done from over 9,000 ft to 1100 ft in less then 20 miles on just a coaster brake. No problems as long as you dont ride it too hard and get the brake hot. On the way down I was hitting over 40mph and had no problems just using the coaster brake.
But.... it is not a bad ideal to add a front brake.
Mount Lemmon Ride pictures by FR31MB - Photobucket
YouTube - Motorized Bicycle up Mount Lemmon in Tucson,AZ