Hi,
He says on that link "At the very highlight end of the scale, photographers often don't mind having a perfectly pure white tone in their prints. We call these 8-bit RGB 255,"
I would not agree with that unless it's in a very small area like a spot reflection. I make great effort not to have blown highlights that give 255,255,255
Back the main question:
I have tried levels and curves but they both appear to effect all of the print.
You can easily do it with curves but you need to pin most of the curve to the original straight line. Some curve editors move other parts of the curve a considerable amount when one particular point is moved. The QU editor does this to a small extent so "pinning" is still required.
See the attached example, a curve with the top end flattened in an exaggerated way to illustrate this. I couldn't find an image with a large area of pure white!
The original "white" was 235,244,242 indicated by the red dot on the curve. The filter preview shows the effect where the curve above RGB value 192 has been reduced. In practice I assume you would flatten above say 245 or 250. You would lose contrast in that range of course.
Terry