JJust came back from the event. Just realised no one cared about rules. A lot of people brought their camera inside and although they wrote there more than one years old strictly must have a seat, it was not enforced at all.
After the event and knowing the limitation of 400mm f5.6 (actually I bought it to shoot bird), I was thinking should I buy a 50mm/85mm f1.8 to shoot this kind of show next time. With the 3x digital zoom (no loss of quality from 600D) and 1.6x crop factor, it will become a 3X1.6X50mm=240mm f1.8 or 3X1.6X85=405mm f1.8. Too good to be true right?