I know this a few weeks old but I have an 09 roubaix and I am about to mount the srq on it for my carradice pendle bag. I don't know why everyone is so scared of a carbon seatpost. That seatpost is strong as hell. I'm 99kg and it holds me fine (it held me at 110), if I was worried about a 7 to 10lb bag attached to the strongest part of the tube breaking I'd never ride the bike. A 5lb load on a 400gram seatpost rack is not going to break your bike. But I would choose one with a nice wide clamping area and that wont scratch the post all up.
A lot of distance riders ride carbon bikes and use saddlebags/handlebar bags and I can only guess that some of those guys have to install little seatpost racks to hold the bags.