There is no requirement to extend the grounding electrode conductor from the transformer, but you must have a fault clearing path back to XO. This could be, in some cases, the grounded conductor, or an equipment bonding conductor.
In most cases the drain wires are exactaly that and are not large enough to be used as the required equipment grounding conductor for the primary side of the transformer.
Isn't the absence of a specific requirement to bring the neutral of a SDS to the disconnecting means a big hole in the NEC. If the SDS is grounded at the transformer then the lack of a grounded conductor to the disconnecting means creates a truly hazardous condition. If this transformer were owned by the utility instead of the customer the provisions of 250.24 (B) would require that conductor's installation in the raceway with the secondary conductors. I cannot see how the transformer's ownership can be allowed to obviate so basic a requirement.
Some few of these transformers can be reconfigured into delta secondary configuration. If that is possible in this case the transformer and the circuits it feeds could be readily changed to a corner grounded delta system by changing the connections and installing listed fuse shunts if applicable. The handbook commentary makes it clear that 480/277 wye connected systems are particularly prone to destructive burn downs from ground faults so if the user does not need the 277 voltage for their application there is no good reason to install the system as Wye connected.