In my opinion, and there are many more experienced, I think you will need two servos, one controlling a sheet plus a line from the yard end, and the other likewise. I base this purely on my knowledge of barges and thus handling a large spar with one end up in the air. Lines to both top and bottom of the sail on each side will keep it a better shape than if you just have a sheet. At the rate you work you must be nearly ready for doing this!.

Cheers, Nerys