Is the zipper metal of plastic? If it's metal it is probably prohibited as that can scratch a slide surface. If plastic, I don't see why it would be a problem.
Well, metal is prohibited in general, even on the front. They may or may not call you on it. If the life jacket is covering it, they may not even notice, though is it worth the risk.
For what it's worth, I actually kinda like it for water park touring if it had a plastic zipper.