I don't beleive you are required to ground (bond) the yoke. If you look at 406.2
(2) Isolated ground receptacles installed in nonmetallic boxes shall be covered with a nonmetallic faceplate.
Exception: Where an isolated ground receptacle is installed in a nonmetallic box, a metal faceplate shall be permitted if the box contains a feature or accessory that permits the effective grounding of the faceplate.
It seems to imply that the yoke will not be bonded and therefore a metal faceplate is not permitted because it cannot be bonded by the yoke.
