There is hardly anything simpler - two sensors, compare the two, and vary OA accordingly. Most systems with Outside Air economizers already include all the equipment necessary to make it work. Only additional controls logic and the sensors themselves are all that's required. There is often a problem in execution, though, and that depends on the controls vendor and their experience. The writing's on the wall, though - it is fast on its way to becoming a standard.
Having said that, I personally would not choose to use a CO2 sensor. Rather, a "general" Air Quality sensor is better. Typically, they read CO2 and a whole host of other contaminants. Strictly speaking, CO2 level is the definition of ventilation effectiveness, but specific CO2 sensors tend to be expensive and finicky.
As with anything "new" technique (10+ years old, at least), there can be unanticipated pitfalls. I have seen systems trip the safeties on equipment because the OA dampers shut on the passing of a truck. That's exactly what you want from an occupant point-of-view, but not by Maintenance.
The controls vendor needs to anticipate such consequences, and ensure that necessary deadbands or delays are incorporated - something often left out without even involving CO2 ventilation control.
