A quick response including 'health and safety reasons' should quickly put an end to that. But of course, there are always some customers that won't like any answer!
I'm the same - 'work wear' for setting up and taking down. Safety shoes in particular, not only for the risk of dropping an item on my foot, but also to reduce the slip hazard whilst carrying in equipment. Any risk assessment and method statement would soon highlight that performance attire is not suitable for rigging a mobile disco.