There's nothing wrong with feeding chickens cooked eggs. As long as it's all smashed up into something unrecognizable, they'll never make that connection. Cooked organ meats like the liver suggested earlier would also work well.
For some reason, commercial feed companies would have everyone believe that chickens are vegetarians and can do just fine with no source of animal protein. There aren't many chicken feeds left that still include it. I think I actually looked at the feather fixer food at TSC and it had none. I...