In this answer, it is stated with some references that the madhahib of sunni Islam agree that every woman, whether a Muslim or a kafir, needs a wali to get married under Islamic rule - I suppose this is with the exception of hanafis who don't consider the consent of the wali a necessary condition for marriage.
This surprised me; I had a vague idea that in matters of family law where no Muslim is involved, kuffar are free to regulate affairs according to their own laws. I'm not aware of any source texts that would call for imposing the concept of wali on kuffar in an Islamic state. This brings up the question:
Question: what is the legal basis for obliging kuffar women to have a wali whose consent is necessary in order to get married?
It might be a matter of 'urf, or of qiyas, or of the dhimma; maybe I just don't know the relevant source texts. I'm not sure whether I'm missing any potential legal sources here, but if I am, please point them out.