Not really clever. Format strings have a fairly restricted format; that one can't have all that many bits of entropy. Once the obfuscation method occurred to an attacker, it wouldn't take long to brute force it even if they didn't have a file to search for suggestions. (And now that the idea is out there, I bet any number of hackers, wearing assorted coloured hats, are trying this out on other systems Even As We Speak.)