Thanks for the answer. I was hoping for something a bit less interactive. LR lets me just hover over a button so see anything at 0 or 255, or I can activate the upper and/or lower button to leave the clipping display on. Very much like a camera viewfinder. Even better would be something like a clipping option on the view menu. It's a way to make a quick check when doing blends.
As an alternative you can add a Threshold layer as the top layer, especially useful when doing multi-layer blends, and by sliding the cut-off point to the shadows or the highlights you can see where the clipping takes place. Granted, not very elegant, but it does the job and works with blends.