I test over-ride rig character with 2.92 version. it seems already improved much. (with driver , shape key, and script generate custom prop with driver etc)
Then If there is custom property which drive shape key, and if custom property already set as over-ridable in library scene (by script, or UI custom property editor) it seems work as default.
And even though we do not use driver, I can still force shape key data as over-ridable in linked scene from outliner.. (on the mesh data shape keys, r-click option, add Hieralchy override, it make shape keys value as editable. (without driver and custom prop)
Then I hope to know is there way by bpy, it can force selected mesh shape keys as over-ridable in library file without set driver. Do someone know function or which can make shape keys as over-ridable, without add new custom-props?
I serched bpy document, but it seems only work for custom property (set over-ridable) so I hope to know if it is current limitation, or I miss something still..