There's no requirement to use a carbon seat post. I use an Al seat post and bars on a carbon frame.
If you mount a saddlebag properly there is very little cantilevering or motion, but it's still nice to keep the weight to a minimum. My Carradice Camper fully loaded weighs 12lbs and is supported by loops on the saddle and a strap around the seat post. I have done thousands of miles with this setup and never had an issue.
This is offtopic but is there anything supporting the carradice camper other than the loops on the saddle and seatpost? I've been tempted to buy one but I thought you need to buy some sort of rack support for the large camper bag. Seems like a great choice for lightweight cycle touring for sure.