I could not resolve this issue. I needed this to attach two objects at runtime in VR.
One should be able to grab a door handle and open the door. With normal grabbing or reparenting this would not work since the unlimited force of the virtual hands would break the hinge of the door.
As a workaround for this issue I implemented some form of spring. One end is attached to the hand and the other one to an invisible sphere. When one would want to grab the handle, the invisible sphere is made a child of the handle.
This way the parent of the door handle is not changed and any spring force induced on the invisible sphere is then transferred to the door handle.
There I can change attachment force to my likins even at runtime.
When someone has an in-depth guide about physics constraints please let me know. I my view they often behave very unpredictably(e.g., the soft constraints ^^).