This is very similar to what I currently do! My rack is made of thin tubing so I wrap rubber and tape around it to make it thick enough for the seatpost mount to grip it. The problem is that most blinky light mounts are frustratingly flimsy and being right above the rear wheel is a particularly brutal place for absorbing road vibrations. So the mount will brake or the light will bounce free... this has been a problem with all of the blinkie lights I have used

Peter White sells the 4D Toplight Permanent which attaches to a rear rack via a strong aluminum strip that you actually have to drill and screw to the rack. My coworker has one and it's sturdy. But it has no blinking mode and is not as bright as the $5 Nashbar blinkie