Personally, I don't think adding an engine braking light is a great idea. As responsible riders, we should be keeping an eye on our mirrors at all times, and when there are cages/bikes behind us we need to let them know we're slowing down by tapping the brakes. I do the same thing in my car.
Like COS said above, I'm in the habit of 'tapping' my brakes to let following drivers/riders I'm slowing down gradually (i.e. engine braking). I let off or partly close the throttle, then hit the front lever very lightly a couple times. I only wish more drivers were attentive enough to be able to judge a change in speed without having to see brake lights.
OT...Big pet peeve: Drivers who believe their foot should be either on the gas or on the brake 100% of the time. It's like people forget they can coast. It's even more annoying in a traffic jam.
Back on topic...During group rides in the twisties I'll tap my front brake to let following riders know I'm slowing a bit ahead of a turn/curve. This is especially true when I'm leading the ride.
BTW, I've read that downshifting to slow down is less fuel efficient than simply engine braking/coasting, which makes sense to me. I still prefer to downshift at times to bleed off some speed.