I gave up. I had a friend who was willing to take her, rubber eggs and all and she's now happily laying her rubber eggs elsewhere. She was very tame and the lady in question just wanted a pet. So all's well there. I am suspecting that it's a genetic flaw of some sort as the barred rock came from the same hatchery as 2 other barred rocks who developed weird issues on me. The rest of my flock (all Wyandotte) lay very strong shelled eggs.
On using crushed egg shells, I tried that once. Shortly after I did I had a sudden problem with hens breaking and eating their own eggs. I've always feared to give any hens crushed eggshells again as I know that chickens have really good color recognition and I'm afraid they will see the color of the shells and decide that's the color of food.