The original question is from 2009...
I'm sure it's possible for a chicken to be force-fed too many vegetables.
But I think free-choice chicken feed plus free-choice vegetables should be fine.
Treats that are high in fat or salt might taste so good that the chickens will overeat and unbalance their diet, but I do not think that would happen with vegetables. Chickens are fairly good at balancing their own diets when given reasonable options.