I fancy this feature so badly on the 5D3
I don't think this feature exists in the HW encoder.
Is the HW encoder the same as H.264?
I think its still DigicV. All preceding canon "CBR" was just VBR with prediction. 5DIII is slightly different but the way it works I don't think real CBR is possible. Even if you set same quality for each frame it will still encode a different rate based on what is happening in scene and settings.
Is this even really possible on other cameras? Seems like they just throw away most of the data to achieve something like solid 24Mbps, etc.