Do we know that's one of her costumes? It could easily be something they pulled from stock as a nod to her character and what she felt she "should" have worn or just something that would fit Staunton without tailoring. I have no idea what their tech schedule is (or when the National's costumes are usually ready) but they have a good five weeks until they open. It's very possible her actual costumes aren't done yet. I don't think they released actual production photos from Angels until a few days before first performance. Before that it was just the individual actors in the pink and blue lighting.