I experimented with the old 550D of my dad. I got a successful clean picture using either USB (canon supplies a "USB Webcam Software", though the 550D is not officially supported it works. I also managed to get clean HDMI out using "magic lantern" (a firmware modification for some canon DSLRs which enables a lot of hidden features, allows clean HDMI, and on a beta branch allows "simulated focus bubtton press" to keep the camera from shutting down due to inactivity)