oh? K?
Not sure what you mean by that, but having had a moment to think, I would say this, the mode of dress for a given event is a complex issue for a lot of people. People get overly worked up about what other people wear to things (like a thong to a funeral). It is what it is, so I would have to say that whatever the answer about OC, it gets into a broader issue of social courtesy of "appropriate dress".
Since, I generally care less about what other people do with themselves than most people seem to, I am not the best person to ask about where the line would be on "appropriate apparel" WRT OCing a firearm.
CCW avoids all those issues.
Generally I would say that if there wasn't so much hoplophobia people wouldn't think twice about a nice dressy leather belt and holster. Since you brought up "olden times", they generally did not back then, in fact in various societies at various times, weapons have been worn regularly as part of formal dress...
p.s. this being a casual affair it shouldn't matter though...