You have lost me, so let me describe more explicitly a modified proposal, based in part on your ideas. I will describe it in terms of electron counts as indicated by A/D convertor output levels. For concreteness, I consider a sensor with well capacity 25,000 (as in the 5.4 micron photo-sites of the Olympus E-500, so probably typical of current SLR's)
A/D output corresponding to 25 electrons or less: set to the same level as if no electrons were detected.
A/D output 25 to 25,000: scale linearly to the range 0-25,000, so 25->0, 25,000->25,000 and the slope in between is 1000/000 So roughly
25 -> 0
That's actually quite unnecessary. Losing 25 out of 25000 electron counts reduces highlight headroom by only .0014 stops.
It seems to me that noise induced fluctuations between nearby pixels are amplified by the factor 1000/999, so essentially unchanged. Is that your point?
Not exactly; my basic point is that raising the blackpoint is just like moving a pile of dirt from one location to another, adding more dirt to it, and erasing the original location. The noise isn't "down there", per se, to be clipped away. The noise is everywhere; at every tonal level. In an absolute measurement, noise increases at higher tonal levels, but generally decreases relative to signal. When you take a signal that used to be above black, and make it black by clipping to 0, you have a new black with a higher level of noise than what used to be the noise level at the old black. The more you raise the blackpoint, the more sharply the S/N ratio drops in the range immediately above the clipping point. The only time that simply raising the blackpoint works is when you raise it to a tonal level where no signal will be directly above it; then there is no signal to experience the great loss in S/N.
This raises a perceptual question: when looking at very dark parts of an image, but with eyes adapted to the overall luminosity level of the image, does the detectability of the noise fluctuations depend on the ratio of fluctuation size to the luminosity in that dark part of the image, or relative to the overall luminosity, or something in between?
It seems that it exists, to an extent, relative to the scene, but things outside the frame have an effect, too. You're going to see more shadow tones and noise in a dark room, in an image lacking in highlights, full-screen, than you will with the same levels with bright highlight areas, or in a window on a white desktop.
For the levels in your example; 25 electrons at ISO 100, I doubt you will see much of any change when you move the blackpoint, if you are not pushing the exposure index in your render. You have to use the Shadow/highlight tool or something like it, agressively, to see such a change. 25 electrons is only 2 to 4 ADU at ISO 100 for DSLRs.
If there really is a problem here, my next idea is simply increasing the amount of spatial averaging (noise reduction processing) done at low levels, so that at 25 electrons or less a lot of resolution is sacrificed to avoid visible noise.
That should work; I don't know why more converter don't do something like that; it seems that they generally soften their highlights, too, when you apply agressive NR. While waiting for the feature, you can can render one conversion with sharp dtail, and one with lots of NR, and use a luminance mask to apply one over the other.
All this based on the idea that below about 100 photo-electrons, S/N is less than Kodak's "minimum acceptable" guideline of 10:1, and this should only ever be the case in deep shadows below the level of significant detail. (25 electrons is about 10 stops below that maximum signal of 25,000 seven stops below mid-tones at base ISO speed of say 100, so a good three stops below mid-tones even at sixteen times base ISO speed, or say 1600.)
Well, with most current DSLRs, 25 electrons of signal at ISO 100 is only 2 to 4 ADU above black. Your 25,000 electron ISO 100 would be about 4, so lets use that. The read noise is much stronger than the shot noise there; read noise is about 2 ADU for a typical DSLR at ISO 100. That's about 12 electrons, so you have a signal of 25 electrons, 5 electrons of shot noise, and 12 electrons of read noise. That's a total noise of about 13 electrons, only 1 electron stronger than the read noise itself; the shot noise is almost totally irrelevant, as the read noise predominates by a wide margin at this level. The lowest ISOs on DSLRs are mostly crippled by read noise, not shot noise, in the shadows.