Just a thought....couldn't a small amount of soap batter that is at a thick trace be used to 'glue' the pieces on?
Say maybe use the spatula scrapings from the end process of making a soap batch to quickly glue on embellishments to the face / tops of the bars from another batch? Good way to use up small amounts of wastage as well, rather than leaving those bits in soap pitchers (especially if there's not enough to fill any cavity molds), I would think.
Or just mix up a very small batch of soap for the purpose of gluing embellishments onto already made bars?