I have made some surgery demonstration dolls for a plastic surgeon that showed the process of breast reconstruction using skin that is still attached to muscle and how it is left intact as it is move for the reconstruction. It had to show the incisions and the tunnels under the skin where the muscles are threaded through with skin still attached. This was a similar problem to the OPs, I believe. What helped me sort it out is that I used foam backed fabric which was able to hold it's shape without stuffing. You can attache the foam to the fabric you are using with low melt adhesive sheet that you iron on, bonding the foam to the fabric back. There are also spray on adhesives but you must be sure they are compatible with the foam you are using and you have to wait for them to dry. Also quite toxic to inhale when you are spraying.
There are dolls on the market that are filled with parts that are removable, Like Ned's Head and the Lady that swallowed a fly-. Maybe take a look to see how they are constructed?
Old lady that swallowed a fly-http://www.hokeypokey.com.au/images/...9931-gr271.jpghttp://timberrifictoys.com.au/images...h%20belly).jpg
Here's a doll with removable organs-http://educationalbeginnings.com/con...330031-big.jpg