I don't understand why a single sentence in a book published 94 years ago in London should be construed as license to disregard the norms of "highland" dress for ladies. There are accepted customs for how people should dress, and wearing the sash on the "proper" shoulder is one of them. These customs actually are "rules" for military wives, and for ladies in the RSCDS when dancing.

Will anyone say anything? Well, in Scotland, probably not.-- That would be boorish. But you can bet your bottom dollar that it will not escape notice, nor will it go without comment. Especially by the "sows" who play by the rules.