This is a tool I made for reading in Renderman renders when relative pathing isn't possible with your folder structure.
The tool features a "Refresh Render Layers" button that looks at your shot render directory and lists the available render layers based off of what has been rendered already. Then the user can choose from the list which layer they would like to read in and press the "Setup Read" button to read in their render. 
There is also check boxes to read in separate crypto reads if this metadata is not stored in the merged exr. Lastly the "merge type" adjusts automatically for filtered and unfiltered (denoised and un-denoised) layers.