The short answer is ‘possibly’, but not because it has a double-sided pull zipper.
Since most body bags, at least in the US, are made of plastic, it wouldn’t be too hard to work the zipper down from the inside with a fingernail or to punch a hole in the bag. However, some body bags come with locks (I don’t know why), some are made of heavy duty plastic or double lined in order to control contamination. In the cases where one of the latter types are used, I seriously doubt that the body is still alive.
If you are considering using a body bag for a Halloween costume, consider turning it inside out so that the zipper is on the inside. :)