Still continuing with my experiments trying to get the best I can out of this very simple single light setup. It seems that as long as the movement in the background video clip does not involve all of the frame the video will still play well and retain facial detail when uploaded to and processed by YouTube.
The chroma key was performed with simple small green screen and single light located above the video camera at about 30 degrees. In this clip I experimented with altering the background "focus" using a changing gaussian blur on a low setting to try to give the impression of a shallower camera depth of field. I think the best "depth of field" effect is achieved with a very small degree of background blur but it seems to work nicely if the shot starts and finishes fully sharp.
I also experimented on this clip with a shape mask on the subject's face to slightly alter the colour without changing the rest of the scene. The video is best watched on YouTube at either 720p or 1080p.