I think it will be quite difficult in real terms to measure core strenght. the concept of core looks at how functional stability can be achieved by a combination of forces one of which is muscle. measuring an individual muscle strenght would not be valid.

I think perhaps a function task can be set and calibrated pre trainning (pilates classes for a set time) and the level of individual achievements assesed (based on calibration.)

Just thinking about that, you can do a comparative study btw pilates and another form of exercise on a functional task that demonstrate core muscle effectiveness.