I'm finding that food stuffs yield smoke flavors very similar to their taste. examples being mostly whole woody spices like cinnamon, allspice, and star anise.
It helps to put stuff like this in a foil packet so it will smolder some before being consumed by hot flame that will burn up any volatile flavor chemical before it reaches your meat.
If you're a bit nuts like me, you can test out the smoke flavor by making the foil pack, burning in on the stove, and putting some neutral food (cooked rice) in the way of the smoke plume. Try it with star anise, its amazing (not something I'd normally enjoy eating but really cool).
I don't see much potential for spent grains though. I'm not sure what a grain flavor would do to beef or pork. And I have a simple rule I cook by: use each ingredient in a way that will best suit it. Going by that rule, I'm thinking it might be better, in terms of final flavor, if you add the spent grains to water and salt, and cooked yourself a beer brine.
better yet feed the spent grains to a grass fed cow and fatten her up.