If it's the typical inexpensive CPSC-mandatedone-piece plastic unit you are out of luck, they are glued together. I suppose you could cut the reflector off of the mount and glue a large safety pin to it and pin it to your bag.
A better idea might be to get some kind of flashing rear light to put there instead. I use a Planet Bike Superflash SuperFlash but there are tons of others out there.

If you feel you need a reflector or rear light you probably should consider a headlight as well. I got these Blaze 2 Watt as combos with the rear lights for a decent discount.