It's still quite simple just to make your own OnTarget() method as well as GetError(). Even though it is more redundant, it should not lock the thread like it will otherwise. Simply, use GetSetpoint() and PIDGet() from your PIDSource object to calculate the error. From there, you can tune your OnTarget() error. For example...
Code:
//PIDSource is called source, PIDController is called controller.
float error = controller->GetSetpoint() - source->PIDGet();
float tolerance = error * .05; //any percentage value
return fabs(tolerance) < 5; // a tuned value