Ok, so I think there are several ways of doing this, but I’m not sure how to go about getting it done. As usual, the blender wiki talks about it being possible but fails to actually show how it’s done. I think I need a separate render layer for the object I want to apply the filter, but I have no idea how to move the object to the new render layer (and no, it’s not “M,” as that just changes which scene layer it’s on, and as far as I know, you can’t have a filter for different scene layers).
You can isolate objects by giving the object an object pass index (Object / Relations panel) or its material a material pass index.
Enabling the Object Index or Material index pass in the render passes panel allows you then to isolate the object in the compositor to apply specific filters.
You could also just add it to its own layer and set up a render layer just for that layer.
It would be helpful if you supply a blend file and screenshot showing exactly what you want to achieve as otherwise answer may just be general principles rather than a specific solution in your case.