I see your predicament. They need to understand that the way it works is that they ask the chef and his staff for their take on "such and such." You make a couple of versions after service one day, let them try it, and then sit down to discuss where that dish might be headed.
Maybe do it anyway. Make your version, put it on a plate, and let the ball be in their court. If they don't feel like trying it, have it for lunch. :-)