I have done that. I have attached the revelate designs terrapin:
https://www.revelatedesigns.com/index.cfm/store.catalog/seat-bags/Terrapin
I had it already, and having spent £toomuch on my bike, couldn't afford anything new for luggage. I made a frame by bending some aluminium tubing (heated on the kitchen stove) into a "saddle shape" and attached it with rubberised p-clips to the headrest. You can see it here
http://cruzbike.com/forum/threads/radical-designs-banana-racer-panniers.8674/. The big pole is for lighting, and not needed for the bag to be mounted. It was good, and didn't noticeably affect the handling. The suspension effect of the cantilever seat design was "different", less bouncy, with it there and weighted, but not really worse or better. I don't have it mounted any more.
I think however the radical designs banana racers hold nearly twice as much by volume, are easier to fit, but are more expensive. I'm not sure which is more aero.
