I haven't seen any code requirements that the isolated ground MUST originate at a specified point. However it seems that 250.146(D) assumes that you will be running it back to the service (or APPLICABLE derived system) ground: ". . .this grounding conductor shall be permitted to pass through one or more panelboards. . .so as to terminate directly at an EGC terminal of the applicable derived system or service." Often times however, I have seen isolated ground receptacles fed from panels where the equipment ground is the pipe feeding the panel. I just wonder if that defeats the purpose.
