Andrew I'm not really sure how else to explain it, I think you're over-complicating things. The fact that the camera histogram is not linear doesn't really matter. The histogram in ACR isn't linear either.
All I'm saying is that with UniWB, flat tone curve, Adobe RGB colorspace, and other "neutral" in-camera settings, the in-camera histogram will be a pretty close match for what I would see opening the RAW file in ACR with default settings. The in-camera histogram will still be slightly pessimistic, showing clipping about 1/2 stop before the raw data is actually clipped. But for ETTR I would rather have the highlights a hair below clipping as opposed to having even one channel clip.
Bill's point was that the green channel is almost always the first one to clip anyway, so UniWB isn't all that necessary. That's mostly true in the general case; but if shooting a subject where the red or blue channel is strongly saturated (flowers, for instance), UniWB can be useful. It's also useful if you're using a magenta filter to 'cut' the green channel and equalize it relative to the red and blue channels. Since I never shoot JPEG I tend to just leave UniWB on all the time.