I would imagine that you could get yourself a t-shirt with "Security" printed on it, maybe have some sort of badge made up to clip on your belt, and I doubt anyone would give you the slightest bit of trouble.
As I said in another post, I found it really interesting when I was participating in a flea market last month that before hand they announced that there would be armed security about so "...if you see someone with a gun on their side, don't panic it's just our security people."
Later, I did see the security personnel in question. I would say about 5'8", balding, about 250lbs easy, carrying what appeared to be a Ruger P-series in an open "Uncle Mike's" type nylon holster with no retention at all, and he kept rubbing his finger over the hammer. Maybe to make sure it was still there? No way he could see it over his belly. Anyways, nobody was disturbed in the slightest by this open carrier.