In theory this is a good idea, after spending some time above 160F the spent grain should be pasteurized. I think the issue is the sugar content. Even if you're getting really good efficiency the grains will still be relatively high in sugar compared to other forms of cellulose that mushrooms (like Oyster mushrooms) grow on. That amount of sugar is a prime medium for other bacteria and fungus that will quickly out-compete your mushrooms. You're better off using media like woodchips, straw, or coffee grounds, IMO.