In the Curves dialog, click the pencil icon (which puts you into what used to be called "arbitrary map" mode, for freehand drawing), then click once anywhere on the 45-degree curve, being careful not also to drag, which would alter the curve. Then click back on the bezier icon (which puts you back into the regular mode to which you're accustomed). Voila! Now you have seven additional lock-down points that well-cover highlights, quartertones, midtones, three-quartertones and shadows.
George! You dah MAN! Give that man a ceee-Gar!
One correction though, I had to click off the 45 degree line instead of on it. I'm using PS 7.
Thanks a bunch!