You could certainly coat the top of your chimney with a waterproof coating. Unfortunately, the coating will not be able to give your chimney a proper overhang on all sides. You could compare this to building a house without having an "eve" or overhang on all sides; no quality builder would recommend this. The rain and snow coming into direct contact with the sides of the building, whether made of brick, stone, or wood siding would make maintenance on those surfaces an annual event.