Would it be OK to run the grounding electrode conductor for a 25 KVA 3 phase 480-208Y/120V transformer to the existing service ground in the adjacent switchgear? The only accessible building steel is bolted together lattice trusses(not a reliable ground)and the water entrance piping is not in the vicinity? Driving a ground rod would be extremely risky due to existing unknown substructures.

I'd say read 250.30 A(7)
Exception # 2.
2005 N.E.C.

Good advice,thank you. I skipped right over that exception in my quest for the answer. The transformer is fed from a subpanel that is fed from the switchgear. I guess the next step is to verify whether the subpanel is suitable for use as service equipment.
Now I'm wondering what criteria the manufacturer must abide by in having their product listed as suitable for use as service equipment.