Have a look at RawDigger and let us know what you think.
Thanks for the link. I've installed it, had a brief play around, and read a little bit of the information on the web site.
I've already learned one interesting fact. Previously I'd naively assumed that the digital values would reach the maxiumum number that could be represented by the number of bits per channel. e.g. For a 14 bit camera like my 5D Mark II, I'd assumed that clipped channels would have a value of 16383. Looking at a grossly overexposed image (from an HDR sequence) I see that the values actually top out at 15760 for the red and blue channels, and 15761 for the green channel.
Interestingly, selecting a large area that was totally blown in all 3 channels shows that there is absolutely no variation in the blown pixel data - the minimum and maximum values for all 3 channels were exactly the same. I can only assume that the AD converter maxes out at a number slightly less than the number that can be stored by the number of bits. (Presumably it's not analogue saturation, or there should be a slight difference between pixels.) This would slightly complicate the task of finding blown highlights, but all that's necessary is to find the maximum value for each camera, which should be an easy task.
I have to say that RawDigger doesn't look like it would suit my purpose however. It seems to be designed for a deep analysis of files, not for working quickly. It requires you to open the files one at a time, which would be far too slow. It also lacks a way to display blown highlights, as noted in the post above.
I need something similar to ACR that lets you quickly scroll through a list of files and scan each one for blown highlights.