When I was tracking just an object move I would export the file as an animated scene and I could just put a 3D object into the object group and bobs your uncle, the 3D object matched perfect.  With the object group set to rigid motion (aka "mobile" now the group has no animation, instead each point has its own translation keys, how exactly am I supposed to use this?  Simply parenting or constraining an object to one of the object track points does not work since the object is moving and rotating.