The fact that this isn't a "formal" event doesn't mean that one shouldn't dress appropriately. If it were me, I would be wearing a white shirt, RCAF tartan tie, Argyll jacket, and whatever else might be needed to keep my teeth from chattering like castanets. In other words, I'd dress exactly like I would if attending any other memorial service, regardless of its religious overtones. If you aren't going to wear a jacket and tie then the flashes would probably be a safe alternative.