Just take a screen capture of the background video before you walk in front of it. Have a blanket that is one color on the inside (the color people will see) and green on the outside.

I didn't have a green blanket, so I went with red which is more problematic as it matches some skin tones. The lighting was also not uniform, so the red edges show through, and the couch has a shadow.

In PowerDirector just put the snapshot on video track 1, and the movie on video track 2. Extend the duration of the snapshot for the whole scene. Then select modify on video track 2 and enable chroma key. Use the eyedropper tool to pick the green cloak. (red in my case) Tripod is required of course.