"Well, IMO, owners of such guns would probably shoot balls much more than shot. Modern made slug doubles usually will have much shorter barrels, 22" or 24", because they are actually not intended to be used for shot (maybe buckshot)."

Not in the US, they wouldn't! Here we expect our shotgun-only deerguns to group into 2-3" at 100 yards or better than that. Roundball guns simply wouldn't cut the mustard, IMO. Besides, roundball ammunition is purely a handloading proposition and if you are going to go through the trouble to case your own for loading, why would anyone choose a round ball over a proper Paradox cylindroconoidal bullet. Things may be different south of the equator but up here, I am sure that the real thing would sell quite well.