Can you somehow fit the exception in 250.52A1?
Exception: In industrial and commercial buildings or structures where conditions of maintenance and supervision ensure that only qualified persons service the installation, interior metal water piping located more than 1.52 m (5 ft) from the point of entrance to the building shall be permitted as a part of the grounding electrode system or as a conductor to interconnect electrodes that are part of the grounding electrode system, provided that the entire length, other than short sections passing perpendicular through walls, floors, or ceilings, of the interior metal water pipe that is being used for the conductor is exposed.
If you could find a water pipe, and an inspector to allow it... That would be your best bet, other than running wire the whole way.
I did it a few years ago, 50' from the entrance, and the inspector had me chain a sign to the main water valve, and my connection to the pipe. And bond accross the main. I think the sign was something to the effect of "this portion of the water line is being used as electrical grounding conductor". The building has its own maintenance personel. The only reason we thought of it was that we would have to go through some heavy timber beams to get there.
