Don't ask, don't tell...
Rule 1, don't ask, don't tell. If you ask, the answer will always be no. Sometimes after you ask, you may get greeted with metal detectors at the entrance or signage that does have force of law when prior to asking there was no such physical or statutory prohibition. While the zoo may sponsor concerts, I would suspect the prohibition for CCW would apply to the building the concert is confined to and not the entire zoo itself unless it meant the legal definition of an "outdoor music festival."