COPY_EXISTING only searches for files in BIFFs and the game's override folder, but you can probably use COPY together with the USER_DIRECTORY variable. For example: COPY + ~%USER_DIRECTORY%/Baldur.ini~ ~%USER_DIRECTORY%/Baldur.ini~
REPLACE_TEXTUALLY ...
Note that making uninstallable configuration changes to the user's game can be considered uncool. There's a good chance the user won't know how to reverse it.
Restoring an old version of the baldur.ini might annoy people even more as it will erase all settings done by the user or the game in the meantime, which includes the list of played back movies (important if you want to skip them later on), hotkeys, console commands, audio and video options and many more settings.
Maybe you can use AT_UNINSTALL or AT_INTERACTIVE_UNINSTALL to restore the Ini option without overwriting everything else.
Contrary to other game files, the Baldur.ini is changed by the game itself, or directly by the user. If you restore the state at the time of the mod installation, you may as well break something that the user desired.
Activating a feature in a ini file is also common in TobEx. I find it easier to edit than installing or uninstalling a mod component. Maybe there is a way in TobEx for modders to activate something in the ini file from their mod though.
Comments
COPY + ~%USER_DIRECTORY%/Baldur.ini~ ~%USER_DIRECTORY%/Baldur.ini~ REPLACE_TEXTUALLY ...
Maybe you can use AT_UNINSTALL or AT_INTERACTIVE_UNINSTALL to restore the Ini option without overwriting everything else.
Activating a feature in a ini file is also common in TobEx. I find it easier to edit than installing or uninstalling a mod component. Maybe there is a way in TobEx for modders to activate something in the ini file from their mod though.