I haven't had any luck with it but indian paintbrush would be one to try.

This topic comes up on this forum (also Native Plants) from time to time. Most responders say that periodic disturbance of small areas is needed to allow successful seeding of annuals into established prairie plantings. You might try discing or tilling small areas -- or perhaps using some less-aggressive scarification method after burning or mowing.

Although soil disturbance (e.g by bison) was a normal occurrence that allowed native annuals to occur in pre-colonial prairies, you probably realize that your one-acre prairie is more vulnerable to weed invasion. And our modern landscape has lots more weeds (including lots of introduced species) that will readily colonize disturbed sites. So you will probably need to exercise care in how you prepare sites for plantings of annuals, and be vigilant that you don't introduce invaders into your prairie.

Alternatively, you might consider adding plugs of show perennial species instead of annual. For example, there are several perennial Rudbeckias that will give you the same look as annual Black-eyed Susan.

Annual black eyed susan works well for me, though I don'r have a prairie. I collect and strew seeds in the fall, and always have some in my garden.

Partridge pea (Chamaecrista - formerly Cassia - fasciculata) is the most abundant prairie annual in places where the soil is not very fertile. In a planted prairie, they hold their own rather well. Some would say they're aggressive.

Mr. Ownby :)
I decided to try overseeding some annual texas prairie natives this year...thinking they should do well in sparser drier areas of my prairie. I'll let you know later in the year if I had any luck at all :)

Some annuals that occur in stable prairies are parasitic on other plants -- even these conservative species often flourish in somewhat disturbed areas of prairies. Agalinis spp. comprise one group of plants that occurs in prairies. Some species in this group prefer moist areas, while others prefer mesic to dry areas. Species preferring the latter include Agalinis skinneriana, Agalinis aspera, and Agalinis auriculata (also referred to as Tomanthera auriculata). They have very small seeds and may not be easy to establish. Depending on the species, they are usually parasitic on the roots of grasses or members of the composite family. Many Agalinis spp. (False Foxgloves) prefer sandy soil. Another parasitic annual species is Castilleja coccinea (Indian Paintbrush), as another person has already mentioned.

There are also non-parasitic annual/biennial species of prairies. Examples are Collinsia violacea (Violet Collinsia), Hymenopappus scabiosaeus (Old Plainsman), Lactuca ludoviciana (Prairie Lettuce), Polytaenia nuttalii (Prairie Parsley), Proboscidea louisianica (Unicorn Plant), Cleome serrulata (Rocky Mountain Bee Plant), Polanisia graveolens trachysperma (Large-Flowered Clammyweed), Pyrrhopappus carolinianus (False Dandelion), and some Corydalis spp. The seeds of non-parasitic annuals/biennials have a tendency to germinate after there's been a fire. It's fairly easy to get the seed of the Unicorn Plant, Rocky Mountain Bee Plant, and Large-Flowered Clammyweed to germinate (with or without fire), while most of the others have a reputation of being rather difficult.