The Sandhill mixes of brassicas (Rape, Mustard, Turnips, and Radishes) and grain (Millet) could be sown in a special yard for the birds to forage in but the grain would not mature at the same time as the brassicas.
I've grown millet 3 or 4 times and it is an easy crop except that the wild birds will harvest everything in just a few days if you aren't careful. The finches are eating it!! That's your cue to cut it for the chickens - do NOT hesitate!
Chicken lettuce mixes have been around a long time. Wild Garden Seed sells a chicken lettuce mix and a mix of Brassica napus and Brassica oleracea for poultry and milk animals.
Gardening for my chickens - I'm new to this and hadn't given that any thought. I grow a huge garden every year with: sweet corn, beets, tomatos, potatos, okra, pumpkins, squash, cucumbers, zuccinni, strawberries, rhubarb, gooseberries, rasberries, and also have an orchard w/cherries, apples, pears, plums, and apricots. I'll have some very spoiled, fat hens