Riding fixed gear is like driving without brake lights

Not sure what you mean by giving freewheelers "a heads-up" - how does being told that they (or drivers?) may or may not see that you're slowing, in any way protect you?

Have you ever ridden in a fast, close chain gang? My experience is that you pedal hell for leather most of the time, then cover your brakes as soon as you spot any freewheeling ahead of you. I think that, if riding fixed in this context, you should be thinking about calling "slowing" in cases where normally you woud expect the act of freewheeling to suffice.