As mentioned in another thread, leading is relatively easily removed by wrapping strands from a copper scrub pad around a wire brush. I really doubt that a good smooth bore will lead that much as the amount of shooting done by a BP gun is less than that from a modern shooter. I used to shoot trap with my smoothbores and round ball and saw no real difference. Mostly accuracy from round ball is a matter of ball patch thickness and round ball quality. Were you really worried about leading, paper sleeves can be made similar to the modern plastic that would protect the bore against shot scrubbing.
DP