I dehydrate tomatoes, celery, bananas, apples, and strawberries for me and my husband. I have also done onions and blueberries without success. I don't dehydrate blueberries anymore, they crumble into dust. And onions must be dehyrated OUTSIDE, because the smell never goes away!
Dried kale is actually very tasty. When I was in Weight Watchers, one of my co-dieters made it and it was a nice crispy snack. She just spritzed it with olive oil, seasoned it and dried it in the oven. I bet the chickens would appreciate that during the winter.
I think you would need to rehydrate most of the veggies before giving to them. I have eaten to much dried beef jerky and I know what it feels like when the jerky aborbs all the moisture out of your body and swells to four times the size. If you dehydrate greens for the chickens, I would suggest that you don't rehydrate before feeding - they pretty much turn to mush when rehydrated.
I think this is a good idea, and one I will stash away in my brain for when I finally can get chickens. I like having the dried stuff on hand anyway, so I will just try to dry extra!