Out of my lane, but...
I think most guys who're using shotguns tactically use them almost exclusively as breaching tools. For that purpose, the trend seems to be toward a single point bungie sling with some kind of weapons catch. The idea is to keep the shotgun tucked away when it's not being used for breaching, and use a carbine for actually shooting people.
If you were going to use a shotgun as a primary weapon, I don't (at first glance) see why a carbine sling would not suffice. To that end, take a look at Blue Force Gear's Vickers Sling or Viking Tactic's two point sling. I've been using a Vickers sling on my rifle since 2006, and it's the cat's behind.
But like I said, this is out of my lane, and there may well be better options out there that I'm not aware of.