Mid-August would be a good time to plant winter rye in NW WI. It will stay as a small green rosette all winter, and then grow fast next spring. After other plants start to green up, deer will ignore it and it will head out about this time of year. Mixing in brassicas will probably give you more green biomass in the Oct-Nov time frame than winter rye alone (brassicas will not survive the winter). You could plant annual (crimson) clover in the spring next year, and then winter rye (or brassicas) if you like them in the late summer again. It would be tough to establish rye into a perennial clover field. A pH of 6 is not the best for perennial clovers, you would need to lime for best results.
I agree with Hoss on the fertilizer. If you have good soil P & K fertility, I might use some lawn fertilizer to boost the green biomass and protein content of the rye.