Hi RhythmScript -
I did a little bit more research and was able to get a successful portal like effect that you are after even with an additional post process. I have attached the test project for you to take a look at, but here is the overview:
I created an Outline Effect Blendable which was applied via a Global Post Process Volume. This was primarily meant to match sure that the results would work with a blendable like the one you describe above.
Next I lined up my Scene Render camera and the world rendered object, so that the lines of the grid pattern in the world would match up correct.
I set the Scene Render Camera to LDR(with Final Post Process)
I created a Blendable which was simply the SceneTexture:PostProcess0 plugged into Emissive Color and Assigned it to the Scene Render Camera.
This step eliminates the double post processing. You will still have to deal with Texture compression and mip generation, but that was going to happen no mater what.
Here is the final Result:
Thank You
Eric Ketchum
[Test Project 4.10.2][2]