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
SOLVED] Blender Custom Properties and JME User Data - import-assets - jMonkeyEngine Hub
Batch add custom properties - Python API - Developer Forum
Creating and linking Custom Properties in color, degree, float, and Booleans Value | blender 2.9 - BlenderNation
Allow Custom Properties as Inputs for Material Nodes — Right-Click Select
Batch add custom properties - Python API - Developer Forum
Add extra data types to custom properties — Right-Click Select
python - Adding Other types of Custom Properties - Blender Stack Exchange
Custom Properties — Blender Manual
Create custom properties for an object and write them to a text file - Python Support - Blender Artists Community
Custom properties and drivers in blender made easy - YouTube