For a late season plant, I would recommend winter clovers, but since you do not want to plant clovers that wouldn't work for you. Maybe check into the thread about powerplant. I know that it only survives the warm season, til about September, but when it stops growing the food source is still there, possibly for months. Other than that, the sorghum that we plant normally lasts until at least january, that might be an option for you, everything else seems to be demolished before the late season in most of the plots.