I figured out how to use custom properties in shaders and the compositor (Shader AOV). Here's a quick cheat-sheet to get you started. : r/blenderTutorials
Custom Properties - Blender Exporter
Enable library overrides on custom properties? - User Feedback - Blender Developer Talk
Mesh Custom Properties editing through the Blender API
Reading glTF extras (e.g. from Blender custom properties) to X3D metadata, support changing shape collision mode from Blender, documentation improvements – Castle Game Engine
Adding "properties" to objects in a Blender scene - Stack Overflow
is it possible to set all the custom properties of a bone to 0? - Animation and Rigging - Blender Artists Community
Solved] Why can't get the value of Blender CustomAttribute - Questions - three.js forum
Toggle visibility of object using a custom property (Int values) - Animation and Rigging - Blender Artists Community