The highlight protection that's now embedded in the 2012PV takes care of some of the 'apparent' clipping that ETTR may show. So all that need be done is use the tools available to adjust for any additional apparent clipping. Turn on the highlight clipping warning on the histogram. It will show the same thing as using Alt/Opt. Using Alt/Opt on any of the sliders will show the apparent clipping depending on which end of the image you're looking at. Exposure/Whites/Highlights will show it at the top end. Shadows/Blacks will show it at the bottom end. Same as the clipping warnings being turned on in the histogram. It's a matter of what tool should be used to correct it.
I did some tests with a Stouffer wedge overexposed by 2/3 EV as judged by examining the raw files with Rawdigger. The green channels of the properly exposed shots were just short of clipping and the overexposed shot received 2/3 EV more exposure. Opening the raw file in ACR 7.1 with PV2012 and the highlight clipping indicator enabled showed that that step one was clipped. Step 2 appeared intact rather than blown, likely because of the highlight protection. The Alt+exp method showed the same results. The steps of the wedge are in 0.3 EV decrements. Step 8 corresponds to mid-gray which should have a pixel value of 100 in ProPhotoRGB. With the ACR defaults, mid gray is far to light as shown.
Note that white balance is off as indicated by the spikes of each step are out of alignment. Performing white balance with the eyedropper changes the clipping indicator, and no clipping is now apparent. The same applies to the Alt+white method. I think this is a bug and hope Eric is following this thread. In this situation, the clipping indicator and Alt+Exposure methods fail.
Correction for the clipped highlights depends on the image. If the shot is overexposed by 0.67 EV, all tones are lightened by this amount. One should use the exposure control to decrease exposure by the same amount. This provides a linear correction except for the highlights, which are afforded highlight protection. The decreased exposure darkens the highlights and a positive Whites adjustment is then needed. If one attempts to remove the clipping with the Whites control, the midtones are left too light. With this image a negative Whites adjustment of -37 is needed. To reproduce the appearance of the wedge, it is best first to set the midtones with exposure and then the white point.
Shown below is a graph of exposure adjustments. The highlights are rolled off smoothly to afford highlight protection and the midtones and shadows are decreased linearly as expected.
The whites control affects the near whites and has a relatively limited range.
If you want to determine if ETTR is carried out too far, it is best to look directly at the raw file with Rawdigger or a similar tool. The clipping indicator or Alt+exposure gives a reasonable result if the white balance bug is avoided.
If one is dealing with a high dynamic range subject and the midtones of the image are properly exposed but the highlights are blown, one should use the Whites control to bring the highlights down and leave exposure unchanged.