Here are some ideas, although I'm not as far south as you.

Look into mums and asters; those are easy to find and grow. Some mums are forced into bloom before Hallowe'en, so ask the nursery when is that plant's normal flowering time.

Some daylilies are still flowering that late. Also, phlox and verbenas. I had phlox 'David' and Brent's verbena bonariensis going nicely together last October, and I'm in a slightly colder zone than you.

Also, some salvias are late bloomers. And look into coreopsis, russian sage (perovskia), black-eyed susans (rudbeckia), and plumbago.

Look at Chrysanthemum rubellum (or whatever its current name is) - apricot, spreading, shade-tolerant. New England asters, despite their name, are probably better bets than New York asters (the commonly sold smooth sorts), as they don't get mildew.

C. nipponicum/Montauk daisy is also good - shrubby daisy with big white flowers and nice glossy green leaves.

Actually a shrub, but can be grown as a perennial - caryopteris. Nice gray-green aromatic foliage, and clear blue flowers. You can delay bloom by providing night lighting, but I don't know the exact details. (I found it out when a plant consistently bloomed later than its sibling around the corner in the garden!)

As far as daylilies are concerned, look at the known rebloomers like Stella and Happy Returns, but also look at Autumn Prince and its kin, if you can deal with tall plants. AP gets a good 4 feet tall, and has MANY small fragrant butter-yellow flowers. Here in Herndon, it blooms from late August through most of September.

Catmint - Nepeta mussini and hybrids - will be reblooming at the time you want, if you have cut it back in midsummer.

Most of the bonesets and thoroughworts are blooming at that time of year, and the better known family members (perennial ageratum and Joe-Pye-Weed) may also still have color.

I'm not absolutely sure, but I think the various hardy hibiscus should still be in bloom at that time.

That's also bloom time for the Japanese anemones, in pinks and whites.

Mid-September is a little tricky as the true fall-flowering plants wouldn't be at their peak, yet.

Salvia guarantica cultivars would be in bloom then, as would some reblooming irises. My salvia uliginosa was blooming in September last year.

Repeat-blooming roses do beautifully in the autumn and should be putting on a show my mid-September. My favorite one is 'Buff Beauty'. 'Clotilde Soupert' and 'The Fairy,' both small enough for the mixed border, would also be blooming then. Wonder about Peegee hydrangea or 'Annabelle'? Mine were too small to flower last year.

Sedum spectabile cultivars would probably not be ready.

Too early for hamamelis virginiana. But sweet autumn clematis should be gearing up.

I think my lespedeza bloomed in late August or early September last year.