I'm thinking like you, that if the sabot travels with the ball to the target, it may have a stabilizing effect. I'm thinking of running a bolt right through the ball into the sabot to lock them together in flight.
If the ball has windage and the sabot is properly constructed you want the sabot to discard. If attached it must be precisely mounted and if not it will impart pitch and yaw into your projectile.
You only want your sabot to help your ball get down the bore square. If done right the sabot will center the ball in the bore, impart velocity to the ball and and fall away as the ball goes down range.
I am thinking about picking up some delrin, chucking it up in the lathe an cutting a cone in the face. The idea of a cone and not a form fitting ball is so the cone will center the ball on firing, but not full diameter. When the air flow contacts the outer portion of the cone in will push the sabot away. That's my theory any way.
By the way here is a guy listed on Steen's website listed as a maker of Sabots.
WOOD SABOTS FOR SMOOTH BORE CANNONS:
Steve Cameron FHD
292 West Old Andrew Johnson Hwy.
Jefferson City, TN 37760
(865) 471-6640
Something else you might check. Are sabots allowed in the matches you will be shooting? Some do not allow the use of Sabots.