I friend had a single barrel insert on a double shotgun many years ago. I think that he did exactely what you are describing, machining the outside of a rifled barrel to match the internal side of the shotgun barrel. There was a nut on the muzzle to lock the insert in place. Two problems with this approach: first, you get a very barrel-heavy gun. Second, on a double, you have very limited regulation, even if you use excentric bushing on the muzzle to try some regulation.
I am working right now on a conversion project, using a shotgun action as a basis, inserting new barrels and using the original ones as monoblock only.
Have you any idea of what action are you going to use for your project?

Fred