I agree. I don't see the point of buying a dress you like and then hiding part of it. Dresses and jackets aren't easy to get right and most of the bolero, capelets, tuxedos suggested on here as cover ups are generally pretty awful.
This.
f you must, I'd go for something sheer.
My thought was to get a t-shirt in the exact colour and cut it so that it's just shoulders and sleeves, but enough of the torso to keep it in place, and wear it under the dress.
sort of like this but not a shoulder brace, obviously.
KSKshape Arm Shaper for Women Arm Compression Sleeves Post Surgical Slimming Sleeves Posture Corrector Tops Shapewear Black : Amazon.co.uk: Fashion