I am certain this will be a very interesting dissertation indeed! For me luxury is defined on a more qualitative basis and not limited to apparel. For example I believe Apple produces luxury items, just as L'Occitane. The commonality those brands share is a high standard for producing aesthetically pleasing, well made products.
To limit the question to fashion: I think a luxury products in the most pedagogical sense would be limited to couture, bespoke or made-to-measure, and exquisitely crafted accessories and perfumes. For me a luxury clothing item usually carries a justifiably large price tag because at a minimum a portion is hand crafted, but ideally it would be the entire product. An exception can be made if the item is completely manufactured by machine as long as the quality if far superior than other machine made products in the same category.
Examples of brands I consider luxury: Many of the major design houses when under the direction of it's namesake (the most prominent being Cristobal Balenciaga). Hermes, Bottega Veneta, Chanel, Dior (couture), Savile Row tailors, Tom Ford, Patek Philipe, Rolex, John Lobb.
Brands I do not consider luxury: Modern day Givenchy, Louis Vuitton, etc.
I would also like to recommend a book called Deluxe: How luxury lost its luster by Dana Thomas. I am not sure if you have read it, but I believe it would be instrumental in assisting you with your research as it is written about this very subject. Another book I would like to recommend is called Luxury World by Mark Tungate and that is about several brands that keep the torch of luxury burning in crafting, jewelry, watchmaking, automobiles, aeronautics, yatchs, real estate, hotels, art, retail, digital, wines and spirits, food, cosmetology, etc. (I pretty much typed out the whole table of contents lol). I have another book called The End of Fashion by Teri Agins. I have not read it yet, but it may be a good resource for your work.
