I couldn’t identify one of the plants in the Traditional Croplands exhibit at the National Museum of the American Indian, but a quick post on their Facebook page solved the puzzle. Here’s the plant.
It turns out to be Giant Southern Curled Mustard, which GRIN suggests is Brassica juncea subsp. integrifolia var. crispifolia. This is described as an “old southern favourite,” or words to that effect, by many of the heirloom seed merchants I consulted online. There is also an ethnobotanical record online of the species being used by Native Americans, but only as a medicinal, at least in the book I was able to consult. Perhaps it is a relatively recent adoption.