I don't know where you are located, but, can I assume that you want something that will winter kill?  Your only good choices for a legume in that situation IMO is soybeans.  Tillage radish is not a legume and does not make nitrogen.  However, tillage raishes, or other brassicas I understand, will do a good job of controling weeds.  A positive I guess for organic.  Buckwheat also does a real good job of controling weeds, but, you sure wouldn't want them to go to seed and may have to delay seeding them closer to kiling frost (45 days or so).  Buckwheat doesn't make nitrogen either.  Your organic requirments make it hard for me to offer much advise as I don't know the methods to control a cover crop in your situation except for rolling a small grain in the spring.  I'm not sure if any legumes can be killed with that same method?