I'm thinking that while a seatpost failure while riding wouldn't be a good thing it certainly isn't likely to be catastrophic like a fork failure. Also, the carbon seatpost has to be able to withstand the clamp on the seat tube holding it with enough force to prevent the seatpost from sliding down while you're sitting on the seat right? I imagine the force of a seatpost rack would be concentrated more in the top front and bottom back of the mount on the post but if I'm only putting a lightweight bag with some pants and a shirt (no tools, no groceries, no water...maybe a spare tube, a hand pump, and a cell phone but all together I doubt the bag and contents would weight more than 5 lbs.) so it might be OK since 10 Wheels looks like he has that much on his rack.