Select the desired shader and click "Set“Set." ” You will remain on this shader until you select and set a different one.
If you set up a rotoscoping scene with the attribute collection, you will need to break some connections before you can use the PSDR tool. Select the x-ray camera in the hypergraph hierarchy. Go to the channel box. Select Horizontal Film Offset and Vertical Film Offset, right click and select break connections from the menu. If you get the following error message "The attribute 'boneShade.transparencyR' is locked or connected and cannot be modified", then you need to break the connections on the transparency. In the hypershade, select the shader you want to use and open the attribute editor. Right click on the black box next to "Transparency" and select break connections.
Note that this tool works in whatever camera view you are currently looking through.