I agree that the gold bar is there to hold the Koh-i-Noor or a Cullinan diamond. I'm not sure if you're suggesting the Durbar tiara, but I believe Camilla and Charles would be in big trouble if she appeared at his Coronation in a headpiece so connected to Empire. How about the diamond diadem? It is already associated with Coronations. Or perhaps just a tiara like the Greville plus her coronet?
(I'm sorry if this breaks the rules about no rights or wrongs, but appropriateness does have to be considered. Even the diamond diadem might be considered too much associated with queens.)