How about doing one of the RPO's in SILVER? The BR did paint several of the RPO's, as well as other heavy weight cars in silver. I painted an Atlas RPO in silver to match the BR - their model is prototypically correct for the BR - see attached photos. This would not only be another option for us BR modelers, but the silver RPO was included in consists with several other roads in mail service; Rio Grande, Great Northern, Western Pacific, and others.
PS BERWYN; I have photos of the prototype if you want to use them as a reference.