I guess I'm confused by your question. padded bras will conceal nipple show-through in most cases, without any weird bumps or seams from a partial piece being stuck in there. so why not just use a padded bra, or stitch in bra cups?
most fabric stores sell the foam molded bra cups (anywhere from soft to more structured) to be used for support in garments that are strappy, backless, or have plunging necklines. They are usually stitched in by hand, sometimes with a lining between the cup and the dress fabric. You could cut them to modify the shape if needed. Or some people will remove the cups from an existing bra.