I had a minor victory today. I've been trying to make the histograms on my camera and raw converters match and to make the in-camera histogram actually indicate full exposure.
I've been using the Near UniWB jpeg for the D300 to bypass the RGB gains in the camera. I've also selected Neutral and AdobeRGB, but my in-camera histograms were still showing clipping about 1.5 stops too soon. I discovered that the in-camera picture controls are editable and that you have to modify the Neutral setting to minimize brightness and contrast through a somewhat obscure user interface. Once that is done, the two raw converters I'm using needed a default exposure setting of -1/3 eV before the histograms looked good. I now have an in-camera histogram that actually works for the first time!
I don't know why the camera manufacturers don't offer a raw RGB histogram option. It's obvious, it's easy, and it's useful.