I think a single slice divided up between 9 chickens isn't going to dilute their nutrition too much. I think it's adorable, and all anybody can worry about is the extremely remote possibility of malnutrition from a slice of bread once a day? Forrest for the trees folks. Let's not get so caught up in minutite that we forget to take joy in our hobby.
Mine often get the heals of the bread loaf. It is one of their favorite treats. As long as we keep it treats it is fine. I think your hubby is sweet to feed his ladies. Mine thought it was cool the first time he went outside at scratch time and they chicken mobbed him. He loves the evening scratch time.