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My solution for implementing a closed kinematic chain manipulator (in my case a five bar mechanism) was to write the URDF as two serial manipulators. I "close" them myself with my own fwd/inv kinematics equations. In other words, there are four joints, but only two DOFs, and so while the left manipulator has j1 & j3, and the right manipulator has j2 & j4, we can write two of the four joints in terms of the other two, and only really care about two of the joints for control. You publish the joint angles of all four joints, and rviz, etc, are all happy.

Steve.