Even before being born Mobu 2009 with built-in dynamics, I practiced in the creation of such constraints, but with the use of physical engine Newton. Of course this was nothing but the tests for fun, but would like nonetheless to share with you some results.
Basic rigid bodies constraint
Joint constraint demo
Generally engine Newton has a good potential and even the rumor of the withdrawal of its second version, which will perform calculations on the GPU, which in turn will substantially increase its productivity. It should be noted that this engine has a built-solver for ragdol and water simulation technology.
Official site of the library physical simulation, Newton – http://www.newtondynamics.com/
Here you can download plugin source code (Newton sdk is required)
If anyone interested in participation and development of this project, contact me.