I don't see a rule about controlling transformers, so 14-400 should cover it.
14-400 Control devices shall have ratings suitable for the connected load of the circuits that they control and, with the exception of isolating switches, shall be capable of safely establishing and interrupting such loads
I wouldn't think the transformer is the load. Rather, it has a load. If you connect only capacitors to the transformer, doesn't the primary have a leading power factor like a capacitor and not a lagging power factor like a motor?