I see two possible ways of doing it:
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The quicker: define a template for base.json in your package, and bind nethserver-httpd-update event to re-expand it when the RPM restore its original state
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The smartest: add another .json file (being template or not) under the same dir with additional rules that refine or override the rules from base.json.
In both cases be aware of the bug/limitation of PolicyRule I said above: