It would be nice to be able to use ...\arduino-x.y.z\hardware\platform.local.txt file for customization (not only in the hardware related directories).
My goal: one place to define hook-scripts (recipe.hooks.sketch.prebuild) that search in the arduino sketch locations for predefined scripts and execute them. e.g. textually insert some text before compilation (https://forum.arduino.cc/index.php?topic=586019.msg3986846#msg3986846).
If in combination with Global system platform.txt arduino/arduino-cli#999, even better.