Stro if you pony up the money and get the CE rated back protector it is flexible and very comfortable.
Try a Sastec back protector. It's a non-newtonian-fluid-like material (like silly putty): soft and pliable when bent slowly, but stiffens up when bent quick. You can move around with it on your back with zero restriction, but fall down/have an off and it becomes stiff and spreads the impact over a large area - just like armor is to do. I use it outside the normal "cheap" foam that comes in my gear. Cheap ($40), easy to cut/trim to size (sharp knife), and really VERY effective.
I have the Tourmaster Flex series and really like it. It has a mesh inner jacket for summer and a solid cortex shell as well as liners for warmth. I believe it is water resistant and not waterproof but with a taller windshield I would not be concerned. It also has a back pad, shoulder pads and elbow pads. I went down wearing a Tourmaster mesh jacket with pads and it held up well. No injuries to my elbows or shoulders. The mesh did not tear.