Bill, I was looking into DCRAW's source code but I feel a bit lost to answer this: DCRAW uses Camera -> XYZ matrices (provided to Coffin by Adobe), and then another transformation XYZ -> output colour space used. In fact both matrices are multiplied in advance to do Camera -> output colour space in just one step.
The question is: is it possible that the exposure level of a given pixel has an influence to determina whether it will fall inside or outside the output colour profile space?
Think of a pixel in a scene captured with RGB levels=(10,22,45), if it was shot 1 f-stop more exposed, for sensor linearity it would be coded as (20,44,90).
Is it possible that (10,22,45) when converted to the output colour space falls into sRGB gamut, but (20,44,90) gets clipped in sRGB?
I want to believe then that if (20,44,90) clips in the output colour profile but (10,22,45) doesn't clip, is just for saturation (luminosity) reasons, not because the Hue determined by the pixel of the scene can alternatively belong to or not belong to the output profile gamut. Right?
If the answer to all that is yes, it means the degree of exposure of the RAW data will affect the possibility of converting levels on a pixel to a given output colour profile, so the Exposure slider in ACR is a good tool, not only to cancel the effect of the white balance, but also to avoid clipping due to excesive input exposure.
Am I right?
Yes, the exposure can affect saturation clipping as shown in this example. This composition was given two exposures differing by one f/stop. With the greater exposure, the red channel exhibits saturation clipping in sRGB and this is eliminated by giving one stop less exposure. However, the picture is too dark. One can achieve the same effect by using -1 EV exposure compensation in ACR (not shown). In this case, it is better to use ProPhotoRGB in the properly exposed image.
Clipping in red channel
Clipping display in ACR
Image with 1 EV less exposure in camera
ProPhotoRGB, proper exposure
Examination of the raw file of the properly exposed image shows that none of the channels are clipped in the raw file. The RGB multipliers for white balance are RGB 2.648, 1.0, 1.356 as shown by DCraw.