Given that cyclists are not accustomed to looking for brake lights on other bikes, I suspect it would do no good. I am trying to imagine myself behind such a bike, and even if I knew the brake lights where there, I would just not really be in the habit of reacting to them.

Alternatively you could just scream "WATCH OUT!!" every time you hit the brakes. Make sure your group has a forgiving sense of humor though. I used to know a guy who had perfected his impersonation of the sound of an air-leak in the tube/tire. Maybe that would be effective here somehow.

jim