There are structures, such as steel-reinforced concrete overpasses and parking garages, that can withstand F5, so hypothetically you can build things that, if hit by an uber tornado, might be able to measure damage caused by winds higher than F5 level. The trick is getting lucky enough to actually have something that powerful actually hit something designed to measure it. Talk about needle in a haystack.
I want to nominate the Mulhall tornado from May 3, 99. I have heard from a few people that it was doing things along its damage path that they had never seen before, but which didn't fit into standard classifications, so it only got an F4, but it may have been bigger than the Moore tornado.