Dynamically create functions for AccelStepper that use Adafrut_StepperMotor.oneStep methods?
I ended up just inheriting AccelStepper, and extending the 
AccelStepper::AccelStepper  (   uint8_t interface = AccelStepper::FULL4WIRE,uint8_t pin1 = 2,uint8_t pin2 = 3, uint8_t pin3 = 4, uint8_t pin4 = 5, bool enable = true ) constructor to add my own Adafruit_StepperMotor instance variable and using it in an overridden step0 (which is the only function those callbacks were used in).

class MyAccelStepper: public AccelStepper
{
   public:
       MyAccelStepper(Adafruit_StepperMotor *myStepper):AccelStepper(0,0,0,0,0,false)
       {
         //MyStepper(0, 0, 0, 0, 0, false);
         _myStepper = myStepper;
       }
   protected:
       void step0(long step){
          (void)(step);
          if(speed() &gt; 0){
            _myStepper-&gt;onestep(FORWARD, DOUBLE);
          }else{
            _myStepper-&gt;onestep(BACKWARD, DOUBLE);            
          }
       }
    private:
       Adafruit_StepperMotor *_myStepper;
};