Doesn't Waterloo have an "airside" subway, allowing passengers to change platform without having to touch out and back in? Admittedly the passenger information down there isn't as good as on the main concourse. Or, rather than touching out, just ask the barrier staff to let you out and back in - offer them your Oyster card to check with a handheld scanner, or at that point they may well just trust you as the easiest option. An automated system like Oyster is going to have to involve some assumptions in the programming, with a fine balancing act between trust and mistrust.