I think I know how to do it! :)

Obviously, it depends on what else is in the scene, but if you place your reflection only figure behind the camera, it will still reflect in the mirror, but will be unseen next to your other figure.
The skeleton is placed behind the camera in this scene, yet is still reflected next to the main figure in the scene. The other problem then is placement in the scene in comparison to the main figure, but that would be trial and error.

The attached picture shows an example of what I mean. A reflected skeleton, yet only Ryan is visible un-reflected.