An interesting idea Dave!

My instinctive reply is that such a tiara would not be flattering to most wearers.

Another consideration is that the diamond diadem works because of the national floral symbols of England, Scotland and Ireland. As such the tiara can be seen as a symbol of the United Kingdom. If a tiara incorporated the royal coat of arms the focus would be on the person of the monarch, not the nation/s. In the modern era, I would think that royal families would choose to focus on national symbols which unify disparate sectors.

One interesting thing in Prince Charles' commentary was that he said all Queens Regnant and Consort since Queen Victoria have worn the jewel.

Hugh Roberts in The Queen's Diamonds stated that the jewel was slightly modified for Queen Elizabeth the Queen Mother; yet, she is the one queen we have never seen an image of wearing the diamond diadem. Given Prince Charles' interest in history and his close relationship to his grandmother, his comment adds to the evidence that she did wear this jewel. I suspect that the fact that George VI's reign was dominated by WWII and the post war conditions may explain why QEQM was never photographed wearing the diadem.