Why not? Go for it. If a suitable donor 12-bore is available. Sounds a possible solution to me.

Can the insert barrels be adjusted for point of impact? i.e. for a form of regulation?

I think plus or minus seven inches is what I'd go for. Would a 1:18 twist rate work?

https://chaszel.com/product/45-70-shotgun-adapters/

Using inserts to make an insert double rifle. No reason a Howdah length 12-bore couldn't work. Shooter loaded trapdoor level .45/70 loads.

https://forum.accurateshooter.com/thread...-rifle.3749835/