Good science means you can repeat the results. Today I ran the HDMI out of my 60D into a MXO2 and used Matrox's Vetura capture software to grab the video output to ProRes 1080. That worked as expected and I grabbed a short clip, opening and closing the iris to get a range of exposure values. The shot was nothing special, just the MXO2 sitting on my desk.
In this shot which looks about right for exposure the scopes show something very interesting, I've got whites at 109 IRE! However, the clip from the camera is only 100 IRE, hard clipped.
With this info in hand, I'm going to try to dig up a chip chart and run some tests to see if the camera is compressing its hot signal down to 100 IRE compressing the range, but still retaining highlights values, or if the codec is simply hard clipping everything over 100 IRE out. If the codec is hard clipping information, that means using the camera's video out for monitoring even with a scope is not accurate. You can see it on the monitor, but not record it to the card. This would be seriously troubling if it proves to be the case which I am suspecting will be true.