No such thing as Co efficient of braking action exists, the Co efficent of Friction, Mu (μ) can be measured, based on this the aircraft manufacturer can give some sort of rough idea of the braking action which may be encountered.
As for Airbus I have been told they just require depth of contamination to assess likley braking action.
Braking action can vary signifcantly due to type of aircraft, technique, speed, weight of aircraft wind dircection / speed etc, therefore no machine has yet been devised to give "Braking action" yet.
A reading of .25mu is regarded as poor friction, .40mu & above good.