Why would walnut be considered any different than poplar or oak? It's supposed to be stored in oak barrels. The bung is not specified. I don't think you can construe a walnut bung as an additive.
Sec. 5.23, Alteration of class and type, is more complicated than it looks. Although "(no) material whatsoever in the case of ... straight whiskey," seems pretty cut and dry, I don't think you can stretch it to include a walnut bung.