`custom properties` set by python as `library overrides` are not saved in file

iam using a simple python script to set some custom properties on objects -
with normal objects that is working fine - with a Linked Library Object it first seems to work but does vanish on file reload.

the script to set it looks like this:
based on the api doc example…

import bpy
bpy.context.active_object["AFRAME_class"] = "clickable"
bpy.context.active_object["AFRAME_show_content"] = "[id*=contact]"

in the UI i can not create a custom propertie for the linked library object.
when i run the script the shows a entry up - but its not editable…

so i just try something that should not work (yet)?

thanks for any tips or pointers in the correct direction!

sunny greetings