Brass is just fine and will match the beautiful brass hardware on my straps.
I have a set of MP military Sam Browne belt and shoulder straps with the nice brass fittings and swivels that all say made in England. If the D Rings are brass it will make it look more like a matched set. I even have a matching holster for the J Inglis Hi Power that would help balance the weight of the DFLE.

The over the shoulder carry looked the most comfortable of the ways DMelone had his rig set up. Even if I don't get to the wilds of Afrika and South America like he does I can at least look the part.