So I figured that if I was missing something in my planting scheme, the butterfly garden at The Natural Gardener would clue me in. The garden features quite a few fall-blooming butterfly plants, including milkweed, fall aster, pentas, dalea, lantana, firebush, and several different varieties of mistflower.
The entrance to the butterfly garden, with a nice little pond. (Note to self, add a wildlife water source to your garden already!)

The right fork. I wish I'd taken a closer look so I could ID that yellow flower on the right.

Farther down the path, firebush, with orange milkweed in the background. I didn't see many butterflies on the red-flowering firebush, and the milkweed seemed only slightly more popular:

Taking the left fork instead, here are firebush and mistflower, clashing like the butterfly-attracting titans they are! I'm trying to keep to a cool color scheme in my backyard, but I told myself that if I had to plant a garish firebush in the middle of all of the pink and blue to feed some butterflies, I'd grit my teeth and do it. In a neglected corner. But where the bleep are the butterflies?

Jackpot! Here's where all of the butterflies are hanging out, the blue mistflower! Hmm...

Upon closer inspection, it became apparent this was a completely different variety of mistflower than the one I've planted all over my yard. This plant forms a huge, tall, dense bush rather than a groundcover under 18 inches. Browsing through the perennials in the nursery looking for a match, this plant was revealed to be the fragrant native mistflower.

This planting of native mistflower was completely covered in butterflies, with butterflies flying a few feet around and above it. I only wish my camera was better so I could have properly captured the spectacle. If you look closely, you can see a blur right above the mistflower that's actually a butterfly in flight. They were a busy bunch, and it took some patience to get a picture where they were in focus.
I also noticed that despite there being both native and Gregg's mistflower in the butterfly garden, the butterflies were ignoring the Gregg's here too.
And last of all, a planting of native fall aster behind a stone bench:

I grabbed a pot of the white fragrant native mistflower on the way out, and I'm hoping I can make enough room for it to fill in and thrill a decent population of butterflies by next fall. If you have a chance to stop by the butterfly garden over the next week or so, I recommend it. I don't think I've ever seen so many butterflies in one place before, and it made my day.