Tim, I'm not sure if this is exactly what you're looking for, but in the Curves Options you can select "Show Clipping..." and then when you touch the black or white points on the Curve you get a gamut warning display. Actually a little more useful, imo
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crap, I think that's what you were saying... if it stayed that way after closing PS it would be a different story, but it reverts to off.
sorry not too helpful