How about taking it to a tailor, explain your problem and ask them to open the seam along the inside of the arm, sewing in some velcro on each seam.
Then, add matching velcro to a piece of black material that makes up for the gap caused by the cast's extra diameter.
After the ball, you can then take the jacket back and have them remove the velcro, restitch the seam.
Just don't drape any cloth over the tux or people will be waving at you and saying "Oh waiter..."
