A RAW histogram would be trivially simple to implement. You wouldn't need to do any Bayer interpolation, white balance adjustments, or any color processing whatsoever.
What's not to like?
I was thinking along the same lines. What Jonathin says is quite true, but I'm not certain how useful raw histograms would be.
On the Adobe Camera Raw forum I once suggested that a raw histogram would be useful, but Bruce Fraser said that it would not be very helpful because, with linear data, all of the data are to the left of the histogram. I wasn't convinced until I did some testing.
Here is a shot of a red flower in which the red channel appears blown as shown in the ACR preview with sRGB:
However, with ProPhotoRGB the red channel is no longer blown:
Here is the raw histogram from the DCRaw conversion:
The raw histogram shows that no channel is blown, but is indeed hard to interpret just like Bruce said it would be. Comments are welcome.