I would leave the corn out of the mix if planted late. The soybeans can and WILL still produce a decent crop and will indeed stay green weeks later than traditionally planted soybeans. I've tried this later planting myself and had impressive results. The only problem I had was that the decreased bean yield was hardly enough to sustain many deer after the local Ag crops were picked. It seemed once the corn came down in my area then my bean plot got smashed hard until it was nearly gone! I would suggest planting JUST the soybeans and then sometime in late August, broadcast a brassica or wheat/oats blend over it. This way you will have a consistent food source well after some hard freezes. It works! Good luck

Here's an Oct.2 soybean plot photo showing the heavy browsing. Note that at this early stage the deer were still hammering the leaves. They actually saved the bean pods until late Oct and into early Nov after they had been hit by a few frosts.