I know it runs around 30% protein and if it isn't dried it can spoil quite quickly. Not meant as a primary source of feed for chickens but more of a treat. I need to do more research on it myself. Definitely following this thread!
Farmers have used spent grains for years. As far as how much nutrient is left, I really can't say. My understanding is that there is a lot of protein left, but the carbohydrates have been removed. So, there wouldn't be a lot of calories. When I have access to it, I use it as a supplement, giving them less than 25% by volume, making up the remainder with fermented layer pellets. The birds love it, but you have to either thoroughly dry it, or in my case, freeze it, and thaw out what you want to use before using it. It might be a nice option to offer it frozen during heat waves in the summer.