It's not just poisonous compounds that are a concern, it's also compounds that can exacerbate a nutritional deficiency or impair uptake of certain nutrients. It's been my experience with the goats (chickens aren't free ranging yet) that if there's enough variety, I don't have to worry about poisoning. They nibble a bit of this and a bit of that and keep on moving, so they never get enough of any one thing to cause a problem.
When it comes to feeding chicken to chicken, I'd proceed with caution. If you know that the dead chicken was healthy and the meat is well cooked it will still be creepy, but probably safe. I would never feed store bought chicken to my chickens. There's no telling what pathogens that bird was exposed to. It may be perfectly safe for a human to consume because we can't catch most chicken diseases, but chickens can. Cooking doesn't always ensure all bacteria/viruses are killed.
DC mentioned the Trobrianders of New Guinea. They cook the flesh of their dead and ritually consume it. Even cooked the flesh transmitted Creutzfeldt Jakob disease. CJD is very similar to Mad Cow in its action. There's a very good reason cannibalism is taboo to humans and avoided in most animal species, it exponentially increases the likelihood of infection.
