I think herbs along the perimeter is a great idea. Anything in the mint family would do well. Chickens love comfrey and will peck at anything they can reach, but if you plant the main root out of their reach, they won't kill it. By trial and error, you will find what they will peck at and what they will leave alone. I have found my hens don't bother with: sweet cicely, fennel, feverfew, vervain. They peck at but don't kill: mints, comfrey, daylily. My flock free ranges at will (they won't stay inside their yard) and so far have not killed anything in my garden beds, although they do stir things up and make a big mess.
