Well here's a thought.... how about Siberian pea shrubs which do fix nitrogen I believe and are hardy and produce eatable seeds. Old timers used to plant them around chicken yards as the peas shatter out the chickens will eat them. Our hen house was put up about 90 years ago and it has Siberian peas planted around it. For an annual legume fava beans are not so fussy as beans about good soil and make a taller (3-4 ft) plant.