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.
have you ever considered using an expedition support? also, aren't those slits coming off of the saddle usually made of plastic or something soft? are they up to the task of supporting a 20 lbs load for an extended period of time?