Is it still "cool" to use a "z" instead of an "s" (e.g. "waz" up instead of "was'sup")?

Maybe it could be construed as "cool", but it's basically slang. In fact, both of your examples are slang. In my humble opinion, they are not cool, and show a disregard for the language. What's wrong with saying, "What's up?" or "What's happening"? Those are still very informal questions, but more acceptable in intelligent speech patterns.