I read a very detailed story in a gun rag some years, on the effectiveness of rifled slugs in getting spin. They shot at vertical wires so see it the slug turned in the space between two sets of wires. They didn't turn at all. MAYBE if the swirl cuts were on the nose where air pressure is high during flight, you'd get some rotation. However, if so, I believe it would be minor, and rotation would pick up to slowly to provide an effective improvement to accuracy.
It will not be possible to cast it on in a two part mold, though it could be done in a solid mold with gently slooped and really smooth sides so the slug would release easily. Other than that it would have to be swaged on. Look forward to spending quite a bit of money learning!
