Its strange suggesting using a 2 stroke engine to brake. On my 2 stroke 3 cylinder SAAB, they fitted a device operated inside the car to disengage the engine for downhill so it actually speeded up. The reason being, and this will apply to bikes as well.
Going downhill there is no petroil burning or being drawn into the cylinder so the rings are staved of oil and the engine will seize if going down a long hill. Thats what was noted in the 1961 manual for the car. It also stated to blip the throttle often to keep a good oil supply to rings and bores.
Be aware any old 2 stroke coasting down a hill will not be getting much oil. When i ran 2 stroke bikes, i used to pull in the clutch on long descents