Why would you assume that the construction of the possessive form of a noun phrase must follow the same rule as the construction of a plural?
The problem with doing so in a post-positive noun phrase is that the adjacent trailing adjective becomes the possessed object, as in “attorney’s general” implying that the attorney possesses a general, because that’s how possessive constructions work. That’s why the possessive form of a noun phrase always treats the phrase as a whole.