Thanks for the research tbm.

It looks like the answer is "No. No prop plane has ever broken the sound barrier, whether in level flight or a dive."

I have to believe that that there is no physical law preventing it and given enough power and aerodynamics it could be done, but no one is going to spend the money required to do it. Too bad. It would be an interesting record but that's about it. Not worth much else, I guess.

My thanks to all who posted.