Occlude specific objects from post-process layer?

I’ve created an emitter of GPU Sprites. The sprites being emitted are just flat, really low-resolution images with alpha backgrounds. When I look at this emitter in an underwater volume which has a Blendable post-process layer on top of it, I get a weird chromatic aberration/mirroring effect. This is probably the expected behavior for the post process effect being used, it’s just accentuated to a negative degree by these flat sprites.

My question is, is there some way I can set the GPU Sprites to be “avoided” by the post process volume that they’re emitting within?

I’ve attached screenshots of the above and underwater results of the emitter.

It looks like I may be able to send my emitter(s) to the custom depth layer in order to accomplish this, but I’m not sure how to use it from there. Simply checking “Render CustomDepth” on the ParticleSystemComponent Actor’s Rendering submenu isn’t enough.