A guy in my club was talking about the same idea. Then I thought: What about using water? Have 2 small fuel tanks strategically placed fore / aft, an electric fuel pump and a mini servo to operate the pump switch to fill / drain. Just pump water from one tank to another to adjust the C.G. Connect the vent from one tank to the other, and you have a closed system, and the air has somewhere to go when the water moves. Note the time it takes to transfer the water, and you would know when to turn the pump off. Power the pump with a second battery, or better yet, find one that takes AA cells in the pump. That way, if the pump locks up or something fails, you don't drain your radio battery.

Not necessarilly better than your idea, but something to think about.

John