I am progressing on this. I customized the Cosmetic system with a perspective based version where the controller only adds the Subjective cosmetics (Third person) on the server and the remote clients ( And the owning client if in third person ).
Then there is an ObjectiveProxyCosmeticsComponent that lives on the owning client pawn that manage the objective perspective cosmetics (First Person) with their own list of Child Actor Components.
To setup those to use the related character parts for each perspective, i did it through what i called a PerspectiveCharacterPartCosmeticSet that will declare two related Character Part to apply on both perspective.
Its still in the work so if there is someone interested in this i can give you some updates as i work on it.