pronker tipped me to
WhmZcoat. Think costuming lite, these are designed to go over other clothes. Some are more phantasmagoric, others almost everyday, so you could wear them in a variety of contexts. They can be UNISEX! They can have POCKETS! Custom work to fit ANY SIZE, and the pictures show lacing so they can also be adjustable. The website lists extensive features; unlike most ads, it sells the steak, not the sizzle. Shop there if you don't sew, or browse for inspiration if you do.
I could make something like this. We already have coven tabards that are designed to wear over robes or street clothes, very convenient for designating roles. These are considerably fancier, but the throw-over concept is similar.