Sorry for the late reply. I've been having technical issues due to recent bad weather.
Yes. I will be covering the structure of Variant elements and how to add new ones, but it is rarely possible to actually do so.
Not every item can have new Variants added. A good example is "Women's Pleated Dress Capris" and "Women's Pleated Dress Slacks". While they are virtually the same item, only the Capris can have the Plaid Variant applied. Trying to add it to the Slacks has no effect.
I'm unsure if all that's needed to make this work is adding the Plaid texture to the Slacks' .peg files. I'd have thought there's more to it than that because the game also needs to be told how to apply that texture to the mesh, which could require modding the .cmesh files, which is not possible with current modding tools.
Afaik, the only way IdolNinja could have known about hidden Variants was to find NPCs which use them. For example; The "Police" Variant of "Biker Helmet" is used by the "Bike Cop" in npc_cop.xtbl. The name of the "Police" Variant/Material for the "Biker Helmet" can be found in the Bike Cop's Materials List. It could then be added to customization_items.xtbl (if it wasn't there already).
(Yes. The use of the terms "Variant" and "Material" for different things is confusing.)
Somewhat related to this; I'm also half-working on a separate clothing textures patch, which endeavours to fix some overdone material (fabric) textures and reduce ugly baked-in shadows. It also fixes some Variant textures, like the "Two Color" Variant of "Women's Halter Swimtop":
