Did you watch the linked video? The Google car appears to smoothly come to a stop at a traffic light with two stopped cars ahead of the Google car. There's no "body language" that contributed to this situation, just a following driver who didn't stop in time at a red light with stopped cars ahead. It happens every day.
If you are about to slow down,... let off the gas a little bit
This is actually worse telegraphing than a smooth deceleration with the brakes because by just letting off the gas you start slowing down with no brake light signal to following drivers. It takes more time and brain power to parse that somebody is decelerating when they just let off the gas. At least for me when I'm following you.
If the Google car just enters a slowing-down event, it might be undetectable.
Again: brake lights.