The real problem is how you actually connect this. If you read 300.3(A) it says you can't just string THHN back to a ground point. (not a chapter 3 wiring method)
On the other hand I think if it is suitably protected that is better than having ungrounded outlets.
I have heard of running a new grounded circuit in the accessible areas and tapping off that with fished grounding conductors down/up to the ungrounded outlets through the wall where they are somewhat protected.
I would talk to the AHJ and see what they think.

Personally I would just run new cables for grounded outlets in the locations that actually use the ground. (most consumer products have 2 prong plugs anyway)


Greg Fretwell