Polly, that is such a gorgeous garden! So beautifully composed of varying heights, contrasting textures, color echos, and woody and herbaceous plants- masterfully done! Is that an area you can conveniently water? I'm wondering if a large container or two, like those whiskey barrels cut in half, filled with some kind of heavy flowering annual and/or a tall plant (like perhaps one of your Brugs) would be the ticket? That's veering away from perennials, but annuals are usually at their showiest and best the last month or two before frost, and that seems to be the time it needs color the most.
