Halliday & Matthiessen (2014: 391):
There is one functional environment in which we regularly find Epithet as Head, namely when the nominal group occurs as Attribute, typically in an attributive relational clause. … This type of nominal group (sometimes referred to distinctively as ‘adjectival group’) is unique in that it is normally unable to function as Subject in the clause. … Other than this type, Epithets and Classifiers do not normally function as Head. The exception is the superlative, which in other ways also (for example, place in sequence) resembles a Numerative of the ordering kind rather than an Epithet …