“What a recipe for disaster.”
How so?
If I was running a business I would post it in the lunch room. It would be one of my principle management tools.
If x asks why they are making less than y, I would tell them.
X has no business knowing what Y makes. And if you are so open with your employees, you wouldn't wait until X asks. You would at least on an annual basis, during annual reviews, explain the employee's strengths, weaknesses, areas to target for improvement and set goals for growth, both professionally and for compensation. X still isn't entitled to know what Y makes.