I'm returning to this post from the future as Robert Holmen set the dial to look in here.
I'm happy to report decaling via camera is quite useful in the future.
I'm actually trying to match a frame of video by applying it as a decal thru the camera to a model, and I don't even know if this is possible.
The problem is when you Right Click on a Model instance in a Chor the |New|Decal option doesn't appear. The issue here is (presumably) that you cannot apply a decal to a shortcut...
Assuming you actually want to apply the decal and not just view it through the camera (ala camera projection)... the trick to applying a decal in the camera view is to select the model in the PWS while still viewing your scene in the Chor. Once the Model is selected Right Click the Model in the PWS and select |New|Decal|. Locate your image and open it. Your decal image will appear in the Chor.
Note: The decal will only be applied to the Model you've selected so if you need to apply the decal to more than one object you'll want to Export the Chor as a new Model. To give the effect of decaling an entire scene you can decal a single patch with appropriate transparency or other setting applied. Place it between the camera and the objects in your scene.