Technically it's against their policy so a conductor who is strict about the rules or just having a bad day would have the right to disallow it on the train. My guess is that if you bag it before entering the station no one will investigate it closely enough to verify that there's no folding mechanism in your frame. I'd also suggest keeping it in the bag until you are out of sight before beginning your reassembly. Let's not make it too obvious to anyone that there are ways around the current rules and therefore encourage stricter size limits in the future.