Does it have to be waterproof? When I am riding at night with my messenger bag, I figure that the bag itself would be covering most of my back so the impact of my reflective vest would be minimal. So rather than that, I drape the neon yellow safety vest over the bag with the straps through the arm holes. I tie/ velcro the front shut so it doesn't move and take off. The same concept should work for a backpack.
Another option would be to get yourself a standard pack cover and sew or iron on reflective strips in attention grabbing patterns.