Uston's "The Big Player"
Did you read his book? The way they did it, and I don't have the book in front of me right now, was to signal the two digits separately. IE the left digit was indicated by position of hand on arm. While the book is not handy, I could guess that something like right hand on left wrist = 1x, on left forearm = 2x, left elbow = 3x, left bicep = 4x, left shoulder = 5x. As a computer person I would probably use the fingers of the right hand to indicate the X digit above. IE index extended, pinky extended, two middle fingers folded would be 1001 in binary, or 9. :)
I don't believe Uston used that last idea, but it would be a bit more cagey since your fingers would _never_ match the count so far as the pit crew could see..
Or you could use both hands, two 4-bit numbers. or just 5 fingers (including thumb) representing a 5 bit number 0-31 which will really cover most cases...
KISS applies of course, complexity is bad... The very few I have played "team" with were computer types and binary was easy for them to deal with, and it's natural for me since I work with computer internals all the time.