Not officially, but according to someone on Yahoo Answers, if you jump on the train without a ticket while all the seats are occupied, the conductor charges you the regular price for a reserved seat and lets you stand, since they can't stop the train and throw you off. This trick would not work leaving Nippori because the Skyliner has a separate platform there with its own ticket checker, but it would work leaving NRT and it might work leaving Ueno (though I haven't been through Ueno since the new Skyliner started running so I'm not sure).