I got the basics of the constraint friction working for the hinge. I also fixed an issue with the reporting of joint position and velocity of hinge joints. I modified it so it keeps track of the joint position internally and just uses a delta of the prev and current position reported by bullet. This allows me to wrap the joint position passed 2PI so position and velocity have no discontinuites.

I did run into one serious issue with joint friction. The method I am using for it did not work at all with the slider joint. Setting the motor on, even with a very low maximum force, prevented the part from moving at all when applying a force to the child part. I believe I am going to have to go a different route to get this to work.
Added code for a notification within the simulation when a rigidbody part was added or removed. Also added a method to get the mass of a part and all its children. I was planning on using these to build a custom fix for the joint constraint friction issue. However, I have changed my mind. I am instead going to submit a bug report for bullet and see if I can get the prismatic constraint fixed to behave correctly.
Commented out the code for the prismatic friction. I will need to submit help ticket to bullet to get them fix it.
I believe I have the prismatic switched over to using the generic 6dof constraint.