I like some of the ideas about the seat bags but the engineering worries me.

What happens to my carbon seat post if I hang 10 pounds off the back of the seat in one of the bags that several have suggested?

My seat is 7" above the clamp and the back of the seat is 6" behind the seat post. That means the 10 pound bag has a significant lever arm exerting rearward and downward force on the seat post.

Is that a problem for carbon fiber posts?

Can a metal seat post be used to replace the carbon fiber? Would it be stronger?