It's not worth doing this. Noise and artifacts are already present in the HDMI output, so you aren't avoiding that. And the H.264 files that the 7D records to have a generous bitrate of around 45 megabits per second. In other words, they already look about as good as they can, even though they are compressed. Capturing through HDMI and having to crop the image is a big tradeoff for little, if any, gain.
Marco