Hi a 20/8x57jrs would make a fine combo and i would turn down the old 8mm barrel to fit inside the 20g barrel. In europe the 8x57jrs is getting more and more common in doubles and combos as the hogs are spreading, so there should be a steady supply of brass and factory ammo.

If one would ream out the choke on the shotgun barrel and also the forcingcone, and get a shop to turn the rifled barrel to fit inside the shotgun barrel with the rim and all, it would be possible to glue it in place.