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)
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)
300.3(2) says we can run the bond/ground alone. 300.4, protection must be considered as well.
That's what I meant by 'Preferred' method Tripp.
If you can snake in the ground,(chances are you need to gt all the way back to the panel anyway) A new ckt is just as easy/hard.
