I'm trying to think of the best workflow for creating a motion-blurred person but in a still render. Because I don't have many animated assets of people walking nor want the entire person to have the same motion blur shape (ideally, I want the legs and upper torso to have more than the waist), I'm okay applying the effect in PS after the rendering.
In this case, I just need a way to make the person be its own entity so that it can sit on its own layer in PS.
My approach would be to create one rendering with the person being directly invisible so that you still get the shadows and reflections created by them, then do another 'render selected' of the person so that you have them as a separate entity to import into PS. You then blue the person as desired without affecting the pixels behind it. The shadows from the person would be a little off ditto the reflections.