I don't see any reason to suppose that the beam rack would damage a carbon seatpost. Incidentally, I notice they've increased the weight limit on the latest version to 9 kilos. Speaking for myself, I definitely would not carry that sort of weight on one - but more because of the effect on bike handling than because I was worried about the seatpost.

As for bags, Topeak sell compatible bags that look nice, but are quite expensive. I'd guess that almost any reasonably compact bag could be secured to this with a couple of straps and/or bungees.