I don't know if there is a "universal" communication method, but really, how many signals would you need?
A rap on the right side of the helmet/head - "Slow The ****** Down"...
A rap on the left side of the helmet/head - "I Have To Pee"
A rap on the top of the helmet/head - "That Was A Dumb Move"
Fingers sliding from your waist to your kidneys with increasing pressure - "I'm Falling Off!"
