Culling Mask with Depth Occlusion

Okay, so I've done some working around this to make this work.

The first thing I did was use a third camera. This camera has the same settings as my second camera, but with the culling mask reversed - that is, it's set to render everything BUT the objects I want.

Secondly, I applied a replacement shader on my second camera, which would render everything blank white, with no shading or lighting.

Next, I set the third camera to render to the render texture, and render the scene to it (NOTE: this step I would LIKE to be able to avoid if possible, because it does mean rendering the scene twice.)

Then, I clear the render texture using GL.Clear, clearing the colour buffer to black, and leaving the depth buffer intact.

Finally, I render the second camera with its replacement shader. These objects are now writing white pixels onto a solid black colour buffer, but the depth buffer still contains the necessary information.

Now, I have a render texture that contains white pixels where I want the post-process effect to occur, and black pixels elsewhere, which I can use to mask the effect.

This works flawlessly, but it DOES require rendering the entire scene twice. I'm looking into ways I could potentially use the result from the main camera to copy to the second one.