In order to make VR players visible as a full body character for other players in my cross platform multiplayer project, I am currently try to accomplish the same. Regarding rotation constraints of the bones, this should actually work by setting the min/max values for each bone in the Full Body IK node.
My full body VR player is very basic and right now I don’t have any bone rotation limits set. Currently struggling to get the head movement aligned to the VR headset. Additionally I have implemented a similar behavior like the characters in VRChat. So the third person character is kind of a puppet, controlled by the default VRPawn.