I love the idea of having different types of masks. I'm iffy about making it this an incredibly rare and super sekrit kind of item, though. As you point out, all it might take is some pantyhose to become unrecognizable. Instead of making it extremely rare, I'd go with a full-coverage kind of mask being stigmatized -- maybe even illegal -- in city-states. I mean, why would any normal person go around in something as concealing as a ski mask?
Also, along with those different types of masks, maybe we could go with separate description fields for eyes, ears, hair, body shape, and 'other notable physical features' instead of an entire 'masked' description. At chargen, the player inputs, for example, "blue, almondine-shaped" as a description for their eyes, and "short brown" for their hair. When wearing a face-covering mask, all that's seen is "He has blue, almondine-shaped eyes, short brown hair,..." and so forth with the description of his ears and body shape.
It'd be a lot more flexible than an entire masked description.