Actually the NPS standards DO seem to cast the yard lighting in "either or" terms...
"9. New additions, exterior alterations, or related new construction will not destroy historic materials, features, and spatial relationships that characterize the property. The new work will be differentiated from the old and will be compatible with the historic materials, features, size, scale and proportion, and massing to protect the integrity of the property and its environment."
and
"7. Replacement of missing features from the restoration period will be substantiated by documentary and physical evidence. A false sense of history will not be created by adding conjectural features, features from other properties, or by combining features that never existed together historically."
So "new" stuff must be easily distinguishable from the original. Such is the case with the RVC, which must not appear to be some sort of "D&RGW-ish" structure. It must be easily identified as a modern structure, somehow set apart from the rest of the yard.
It seems these guidelines are to prevent "Disney-esque" features from appearing on historical properties. If it ain't historical, it should look that way !
So in my interpretation of these guidelines, the modern light fixtures would be preferable over some light fixtures that look like they are from the D&RGW time period. In other words, fixtures that would cause one to believe they were always there (but never were) are not acceptable.
It would be very interesting to have a discussion with an NPS expert on these standards...