It would be virtually impossible to produce a single number that would encapsulate all of a given board's flex characteristics, but it wouldn't be that difficult to come up with a standard system for rating some flex characteristics. A board's width varies almost continuously from tip to tail, but that hasn't prevented manufacturers from providing a "waist" width measurement that, although imperfect and perhaps differently defined from board to board, still offers very useful guidance to buyers. A "center flex" measurement could be calculated at the board's exact midpoint, or its binding reference point, using a standard amount of torque (the proxy for rider weight) and expressing the amount of flex using standard units. Once riders learned how their own weight and riding style related to the standard center flex rating, I think it would be helpful in making comparisons among boards. A standard "torsional flex" rating might be produced in a comparable way.
Of course, all of this would require agreement among manufacturers on the basic units to be employed, but that's certainly not impossible. Convincing manufacturers that it would be in their interest to do it might be impossible, however.