Well I figure if they eat it, they eat it. If they die, they die. Seriously I wonder if they'd eat anything they didn't like. I have a garden and they didn't eat the tomato plants but were jumping off the ground to get the tomatoes high up. They left the lemon balm and cabbages but ate anything else.
I tossed out leftover rice from school that had cooked onions in it and they either ate it or left it but either way they were fine. I just don't make this stuff a mainstay of their diet. Once in a while I suppose it's ok. OH and make sure there is lots of granite grit to go around and help them work this stuff through their system.
My chickens free range all day. If I think it is something they don't need to eat I will put it in the garbage instead of putting in the scrap pile. You are right they are gonna eat what they want though. They usually come up to the house and eat the cat food too.
I let my Buff Orps Free Range and they get kitchen scraps and what I know they wont eat I just give it to the pigs, Pigs will eat anything. I give them a head of lettuce or cabbage when its really cold or there is snow on the ground. That more or less just keeps them occupied and running around. I also give them pretty much any leftovers except ones with a lot of onions or garlic as this will affect the taste of the eggs.