i believe it is just a variety of millet. I have looked into using it myself, and it is similar to millet, but like you said has bigger heads and seeds. I have never used it myself but if it is like Jap. Millet you will be fine. I have a feeling the ducks will love it. Also, it is legal to hunt over just as long as you dont manipulate it in any way other than normal farming practices.
"If you have to be crazy to be a duck hunter, i dont wish to be sane" Robert Ruark
Its always duck season, there is just a long break from february to september.