Seatpost mounted luggage rack on carbon frame

For the sake of argument, let's suppose you weigh 70 kg, and 60% of your weight is being carried directly over the seatpost (the rest being on your hands). Let's further suppose that you have 10 kg of gear cantilevered out behind the seatpost, with its center of mass being 30 cm away.

In terms of what the frame "sees," the seatpost rack would move your combined center of mass backward by 58 mm. This would be about the same as if you started with your seat fully forward on the rails, and moved it fully rearward and carried the additional load on your back. My guess is the frame can handle that kind of loading.