I would be terribly nervous about getting the luggage onto the ship. The ports are not set up to store luggage, then distribute it to particular ships on specific dates. They could, but I imagine it would add extra costs if more than a few people did it. The piers are not owned by the cruise lines, but governed by the Port Authority in each location. I'm not sure who actually owns the port, but it is not like sending a package to a home or business.