There are a few ways that I would do this:
1. Packages – What plist are you adding or modifying, and do they end up with the write permissions? are you installing them to the English(or whatever language) .lproj?
2. Payload Free Package – this can run a script can issue a defaults write command
3. drop the plist (with proper permissions) into your base OS, then create the image and forget the packages.
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