I'd probably compost them. Not that they would be toxic, but they are pretty high in bromelain, aren't they? I mean, it makes your tongue tingle :0P
Interestingly enough, I found this in the pineapple toxicity section from Perdue: "Excessive consumption of pineapple cores has caused the formation of fiber balls (bezoars) in the digestive tract." These are also called 'mad stones' which used to be used to (supposedly) remove poisons/toxins from human wounds! That has absolutely nothing to do with chickens, but I thought it interesting!
I don't think it would hurt them, really. I am sure it is just fine as a treat. Chickens will eat just about anything. Mine ate a whole bush of habenero peppers without batting an eyelid.