George, perhaps my ignorance is showing here ....
As I understand the trade, every panel has a nice, big, fat ground wire going from the ground buss of the source panel to the ground buss of the fed panel. Even where you have different systems interconnect - for example where there is a transfer switch - you have those EGC's.
All that remains is the mater of size- and I'm pretty hard pressed to imagine a circumstance where this feeder EGC could ever be sized as small as #10.
Is 517 asking you to run an additional EGC? Or, does this requirement address the idea that some might use the conduit of the feeder as the EGC between panels? (If so, then wouldn't the rest of the NEC have 'caught up' to 517 and made this clause redundant?)
Or, perhaps, is this requirement addressing the use of multiple ground busses in the same panel? Personally, I've never been comfortable with relying on the mounting screws and panel case as the only means of connection. If so, then the #10 wire would be 'upsizing' if all the circuits landing there were 15 or 20-amp circuits.
That last condition can't be the OP's issue, as he's asking about panels well separated from each other.
Let me stretch my imagination a little bit further ... what about different systems? Say, the 120v convenience circuita and the 480v mega-machine? Hmm. Those could be from separate services .... bonds back to the service, service bonded to grounding electrodes ... different grounding electrodes are to be connected .... yup, there's the bond, already present.
Have I been in error in believing that you cannot rely on the conduit as yout EGC for feeders? Is that where I'm getting confused? Are folks out there still feeding sub-panels without pulling an amp-sized green wire in the pipe?