A wire is a wire. Surely, it would only be an "earthing conductor" if one end of it is connected to earth?
I know, we're arguing over interpretation, and it would probably depend on how good you were at talking to the inspector, and how stubborn the inspector was, if it ever came to be found!
At the end of the day though, the colour of the insulation makes no difference to the wire itself. The CONVENTION of using green for earth is just that - a convention. I don't see them banning green hookup wire for anything other than earth....
(Ultimately though, there is proper 3-phase wire which doesn't have a green - I don't know how much more expensive it is, but you can buy 25 and 30 metre, 15 amp extension leads here for about $6, which is almost certainly much cheaper per metre than 3ph cord)
As an alternative - I wonder what would happen if you <cough>accidently</cough> wired a 3-pin socket wrongly at the mill. Plug in a standard extension cord.... if you plug the extension cord in any other socket, the green wire would be carrying the "earth" so the lead would be fine. The mill might be earmarked as "illegal" due to bad wiring, but you can unplug it while the inspector is there I guess...
I'm sure this world is getting over-regulated just to keep shiny-bums in jobs. Some of these regs are just red tape for the sake of it!