it would be awesome to have a 'Friendly' tag has clothes, I can't think of other examples except the Hero Kit (dayz Mod) - but there is other way.
example if people wears a YELLOW raincoat it should means your friendly, but i know you can't order to people to behave and wear them only if don't want to shoot people in the face. But there will some (not a traitor) will eventually find nice people....
i know I'm optimistic