Probably makes me a bad manager but.
Was a manager at a place like that once, I used to tell my staff, if you have long hair, use that to hide them. If not, I'd let them get away with hats when I could but otherwise would recommend skin tone headphones. The people who were anal about it rarely got up close to people so as long as it wasn't obvious they were fine. I thought the rule was dumb so I certainly wasn't going to be on anyone's case about it.