Hi all,
I created some small compositor nodes for Blender, where you can control masks based on luminosity. You can check it at link: https://gum.co/lumasks-for-blender
It is free and any CC are welcome. Also I included in the blend file an Orton effect to test it. Search more about Orton effect, it is an effect to create dreamy type of images, mostly work on landscapes but can be helpful on other situations as well.
You can check a small video on how I tested on https://www.youtube.com/watch?v=4clZQ89uEJg