Flight Safety, apart from the use of engine exhaust, what you describe is used in the Notar helicopters - MDD 520N, Explorer etc. Instead of using exhaust gases a fan blows air down the inside of the tailboom to a louvred bin - the size of the aperture is determined by pedal position. An additional, large antitorque force is produced by blowing the same air out of a slot along the length of the tail boom which encourages the down wash from the main rotor to accelerate on that side, producing lift. This is known as COANDA effect.