Well if you have a radio that allows you to adjust the travel of a aux. servo by means of a rotary knob on the transmitter, you could build a sliding tray for the receiver battery.

Then all you do to adjust the CG in flight is turn the knob on the Tx.

I'd use a servo with metal gears for sure, and mount the servo to the tray to act as as additional moving weight, let the servo move and the lightweight pushrod remain stationary.

Think about it... slide the battery pack way back for the flatest flat spins, and then forward when you have to make an emergency dead stick landing.

Hasn't someone done this yet????

You could be the 1st!!!!