Unfortunately, cats seem to go wherever they can scratch up enough "stuff' to cover their "goings". The chickens keep the dusting pile scratched up enough to make it very easy for the cats.
I have cats that like to use the mulch around my hostas, which is pretty disgusting because the hostas are planted up the sidewalk leading to my front door. Not any smell in winter, but when the spring thaw comes it gets pretty rank! I try to stay on top of it.
Is there a spot where only the chickens have access? Maybe make a dusting pile for them in a place where the cats cannot get into.
Short of catching the cats in the act, I don't know what you can do to keep them from doing this. I'll keep on eye on this topic for others' suggestions.
Good luck-I hope someone has some ideas soon.