Square Brackets vs Parentheses in Chicago-Author-Date

That is a fairly common rule. You do this in German as well.

The problem here is that you’ll have to check for context – are we in parentheses already or not. (And: the surrounding parentheses will be part of the main text, not of the citation.) In a plain text system that might be posdible, but I don’t know if that could work with ms-word etc.