While the NEC has rules on the correct installation of an isolated ground, it never requires one. The NEC also never really says haw far back the isolation has to go; to the panel?; to the separately derived system?; to the service? These are questions that are left to the engineer to specify.
In the case of a non-metallic enclosure for a isolated ground receptacle, there must be a means of bonding the yoke provided. If the receptacle cannot be found that has two ground screws, then the box must provide the bond. There are plastic boxes manufactured that do provide a means of bonding the yoke via metallic screw inserts bonded internally to a grounding screw inside the box.
Like everything else, we have to install the right box for the application.
