Two years ago we had a lot of pp ground around this part of the world and the Idea of soys as a cover got brought up a lot.  Never heard if that got approved but it would have been ideal since there was so much extra seed around.  Never saw any pp soy covers though.  We did Rye and it worked really well.  Went in and killed it with Gly in the spring and notilled beans in.  Great seed bed.