Ahh gotcha. Yes as far as I know, this is a limitation in Unreal currently. I am also very interested in the ability to separate passes in a more nuanced way like this, akin to what is possible in VRay, Redshift, Arnold, etc. I like that there are render passes at all, but they are very simplistic. If there is a way to access more detailed buffers, I have never found any documentation on how to do it.
I think part of the limitation is that Unreal folds Specular, Reflection, and Refraction together into one pass. Turning off the specular lighting component in the path tracer would get rid of your refractions, but also would get rid of all the reflections as well. Frustrating.