Grayscaling a layer using two cameras also affects skybox?

I've used the technique described here to grayscale selected objects (by placing them on a different layer): http://blog.theknightsofunity.com/using-multiple-unity-cameras-why-this-may-be-important/

However, I can't find a way to do this that doesn't also grayscale my skybox. It makes sense based on what I know -- the grayscaling is applied to the skybox-cleared first camera image before the color layers are rendered by the second camera.

To be clear, my setup is this:
camera 1:
* depth 0
* clear skybox
* culling mask: the layer with objects I want grayscaled

camera 2:
* depth 1
* don't clear
* culling mask: everything but the layers I want grayscaled

I've tried adding cameras both above and below these to try to render the skybox separately, but it makes sense based on my understanding of draw order that neither of these worked.

Anyone have any suggestions? Thanks in advance!

PS - if it matters (don't think it does), this project is in Unity 2019.2 and I'm not using LWRP/HDRP (and would prefer not to, for now).

 

This is not a direct answer, but have you checked command buffers? Depending on how familiar you are with shaders/image effects etc. this might be another route to achieve similar results. Linden Reid has written a good tutorial about them, see the link:
https://lindenreid.wordpress.com/2018/09/13/using-command-buffers-in-unity-selective-bloom/

 

Cool, thanks! Looking forward to reading this