I was intrigued by the cut of this dress already when the watercolour was posted the first time back in April, but I think our perception might be misled by heavy retouching of the photo from behind (which obviously was the template for the watercolour), particularly in the area to the right of the sash.
To me the black chiffon part of the dress seems to be something like a long shawl, possibly sewn to the inside of the dress around the shoulders but loose otherwise and held in place by the two bars in question. One can see that the black chiffon is a rather narrow piece of fabric and above the actual dress, particularly in the back view.
The elongated leafy pattern appears on the long wide rear side of the sleeves as well as on the back. To me it looks as if either the wide part of the right sleeve was accidently pinned to the waist as well, thus covering the sash, or - more likely - the photographer went too far when retouching the photo in order to enhance the narrow waist of the queen. Particularly the right shoulder looks very suspicious, as does the leafy motif itself.
That said, I do agree with you that the two bars are some sort of three-dimensional clasps to hold the fabric in place, but considering the cut of the dress which wouldn't work without these bars/clasps, I assume they were rather part of the dress than actual brooches.