UEFA rules include stupid stuff like covering up the brands of tv screens around the ground and if you notice the City badge and Etihad sign outside the South stand will be lit up tonight but against Madrid it will be swithed off. All stupid UEFA rules so I can't see anyway we'd get anything displayed that UEFA don't agree with. So I agree with earlier posts where it has to be something ironic or cryptic.
Or maybe evertyone in south stand wearing t-shirts with something on them. That way you'd get in with them worn under coats but at the time of the UEFA jungle everyone wips their coats off. There wouldm't be enough stewards to handle it. Hard to organise I suppose.