You might be confusing motion sensing with strain sensing. Motion sensing wound better measure the damping leading to flutter onset, but strain measuement better measures the stress in a part.
Without a finite element model, though, amd a grount vibration test using accelerometers, strain measurements are only going to tell you what's happeninig at that specific location. You need the FEM to identify the critical locations for a given load case and correlate the sensor output to the bigger picture.
Another way strain sensors could help would be if their output were compared to a fatigue analysis, to give you an assessment of how much life you have left.
Either way, without some sort of structural model, there's nothing to compare it to, and therefore little point in adding strain sensors, at least in my opinion, unless you work for Van's.
Dave