Set mesh opacity selectively at renderer level without changing material opacity?

Here’s what I’m trying to achieve:

I have a scene with different meshes loaded. I want to be able to select a group of meshes, and make these meshes render differently than the others, for example make them transparent.
NOTE: each mesh has a different material and original opacity.

Here’s what i know:

OPTION 1:
I could traverse the scene and set the material opacity on each mesh depending on a ‘selected’ flag. On deselect it can reset to the original material or opacity. I want to avoid this option for different reasons.

OR

OPTION 2:
I could set the Layer of each object and camera, but this completely hides the meshes that are not on the camera layer. I need them translucent, not hidden.

OR

OPTION 3:
Postprocessing.js has a nice outline effect that reacts to ‘selected’ elements, so this may be a clue to how to do it.

Anyone here can suggest a way to do it? Is there a way to set a specific shader on a group of meshes not based on their material but on a layer?

Thanks!