Made some changes to try and get the COM stuff working in bullet, but I have not had any luck. I am going to drop this for the moment and come back to it later.
Switched the base .net code to save out mass as well as density. I also added a boolean to the rigidbody.vb to determine if mass or density is the primary setting for the body. It does some things slightly different based on that value.
The simulator code for bullet now uses the loaded mass value instead of using the density and volume to calculate mass. It calculates the volume directly within each body parts create geometry method.

I also added code to perform a rough calculation of the volume of a mesh.

I resaved out all the bullet test projects so they have the mass saved in them.