I just edited my previous post to add links to some threads about chickens that liked mustard-filled eggs. It looks like you were replying while I was doing it ;)
I have never personally tried the mustard-egg trick, but I have seen chickens eat so many things that I wouldn't make any bets about what a specific hen will or will not like.
Edit to add links to three older threads on this site...
Just putting the fake egg in the run (which already happened) may be enough. They can peck it as much as they want, and never get anything tasty to eat. Eventually they should get tired of stubbing their beaks and decide that egg-shaped objects are not interesting.
I would be concerned that the...