The more directional a blinkie (and the high output ones like the SuperFlash and LD-1000 are very directional) the less useful it is if attached to a bag. I've seen numerous bag clipped-on blinkies that are pointed up, right, etc. but not straight back and level with the ground. No matter how good your plan is to keep it level, sight body position changes, changes in the load in the bag, etc. are enough to aim it wrong.
Best is to mount the blinkies to the bike.
Al