Lightroom: 'blown highlights' should identify permanently blown highlights

Red=Red in just that channel
Green=Green
Blue=Blue

Yellow=Red+Green in both those channels
Magenta=Red+Blue
Cyan=Green+Blue

And white for all three, black for none? Seems that could work well with an alt/option keyboard modifier on one of the sliders.

In fact, this is largely what the alt/option key does today on the Exposure and Recovery sliders, except that it does so on the post-processed image rather than on the raw data. Perhaps if you hold Shift and Alt/Option together...?

Yes it does (the colors and their clipping channels described above).

Where the suggestion becomes useful and interesting is a method whereby you could see that these channels clip prior to adjusting any sliders that might correct the clipping. In other words, you've got clipping that cannot be adjusted so don't bother altering the sliders. I suspect with ProPhoto RGB being used for the saturation clipping, the area where one would want this feature to apply would be only for tone clipping. You see something within the histogram that tells you, don't bother trying to adjust exposure or recovery, the highlights are blown out no matter the rendering.

Black clipping doesn't need this treatment because you should be able to clip there if you wish. But for highlight recovery, especially for those of us that expose to the right, it would be useful to see some indication that the raw data was truly 'over exposed' and clipping resulted that can't be recovered. Those may be specular highlights you don't care about. If one area on the histogram showed you the current clipping, but another somehow showed you what can't be recovered, you could decide to move onto another bracket or adjust Exposure to retain data based on the one histogram feedback and not the other.

Perhaps there is a secondary highlight clipping color besides white. White indicates the current clipping based on the sliders (mostly Exposure and Recovery). Then a secondary color (lets say gray for now) that tells you, these values are clipped no matter how you adjust the sliders.