I'm not aware of any internal blockage they could cause. If a whole kernel of corn doesn't cause blockage, I don't know how papaya seeds could.
Papaya seeds are highly nutritious and high in polyphenols and flavonoids. I consider that good for chickens.
I think it really helps to fully understand a chicken's digestive tract. Everything they eat is aided by the tongue to pass directly into the crop. It is an expanded and expandable segment of the esophagus and can handle all sorts of sizes and shapes of feedstuffs from tiny seeds to mice. Everything then passes farther along into the proventriculus (true stomach) where digestive enzymes are added. From there it enters the gizzard, the mechanical stomach where everything is ground up before going to the rest of the digestive tract.