Since Blender 2.8 have Face Maps already, it should be used inside new Weighted Normal Modifier. This is must have feature and it could help making mid poly assets. Vertex groups doesn’t let you have that much control on bevel edges like face maps.
I want to put this on top again because that’s cool feature to have and I hope someone will look into that 
I will be in dream. I made a screen which shows what I have in mind
Making first screen I forgot to turn on auto-smooth, but overall you should get the basic idea behind this
screen.
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Blender 2.8 has a new modifier called “Weighted Normal” which does his trick automatically. Its not perfect but is quite good, give it a try.
The motivation behind face maps was to use them for widgets creation of rigging addons.
https://developer.blender.org/T51675
You are perfectly, right. It should be used for normals, too.
The Usage paragraph refers to that as Face Associations and Face Assignement.
Face maps were introduced at early state of 2.8 branch before weighted normals.
Weighted normals tools were created while a GSOC in a GSOC branch based on master using a 2.79 UI.
That is the reason why normals editing don’t use them.
The use of face maps for rigging is far from being obvious.
They should accumulate more obvious roles in the future without becoming redundant with vertex groups.
If it does not happen in 2.80,2.81,2.82,… that would not be because of an opposition to the idea but just because of a lack of manpower and existence of tasks with higher priorities.