You can't make this stuff up.
Believe it or not, at least two companies are actually making carbon fiber racks that clamp on seat posts. They both look something like what you see in the photo.
Bontrager and Topeak both claim that their versions of the carbon-fiber seat post rack can support seven kilograms. That's reasonable enough, I guess: If I had a seatpost-mounted rack, I don't think I'd want to put much more weight on it. And, if I were going to carry panniers and camping equipment, I don't think I'd be using any seatpost-mounted rack, whatever its material.
Then again, I don't think I'd be using anything made of carbon fiber if I were carrying much more than a spare tube and a multitool on my bike. Actually, I don't have, and don't plan to install, anything carbon fiber on any of my bikes. Whatever weight savings those feathery tidbits might offer would be negated by the weight my body is storing, as bodies are wont to do when they reach my age. And the carbon-fiber weight savings would certainly be nullified by, say, saddle or pannier bags, let alone what anyone might put in them.
And although failure is relatiively rare in high-quality bike parts and accessories, I wouldn't want to take the chance of breaking any carbon-fiber part or accessory. Carbon-fiber tubes are particularly nasty when they fail: They break along a jagged edge, like a glass bottle. And carbon-fiber edges are as sharp as those bottle fragments. If a carbon fiber seatpost were to fail, having an uncomfortable saddle would be the least of my worries. All that beautiful work Dr. Bowers did would certainly be for naught!
Of course, a broken rack wouldn't have the same consequences. But things could get ugly once that broken rack and its contents fall into the rear wheel. And if that wheel is made of carbon...
Maybe I'm just getting old and cranky, but I still think that fiber belongs in cereal and carbon in filters.