Render single frame and composite with video

Is there a way to have Blender only render a scene/render layer once and composite the resulting render with a video? I’m trying to do a quick test render of a CG/Live action shot and the CG environment doesn’t change during the shot.

Just render to EXR files and composite in a separate scene. (you can set the image node to “single image” so it uses it on every frame). There’s no requirement you use the render layer nodes and work in the same scene your CG elements are in.