That doesn't sound like too bad of an idea. Are you planning on having a front light on your handlebars too? If not, no one would see you from the front.

If you have anything reflective on your bike, it might activate your reflectors too, causing even more visibility.

The only thing I'm worried about is that, since the light would be so close to your back, I'm not sure if the light would have time to spread out. That is, I'm scared you'd have a four inch wide circle of light on your back, rather than a light that illuminates your entire body.

If you try this out, let us know how it works!