The problem I see with that 'solution' is that Murphy sez you'll always have the wrong rounds loaded in the tube for the situation you face.
In reality when you are talking about 'putting pellets into as many zombies as I can per shot if I get rapped by a mob' is there much difference between 6 to 10 inches and 18 to 22 inches?
If your mob is packed shoulder to shoulder in ranks, then in theory you might put pellets on a couple of folks per shot with a wider pattern, but, enough so to gain any tactical advantage? Not so much.
On the other hand, seeing the guy next to, or slightly in front of you, take 9 pellets (or a slug) to the face, might tend to make one shy away from the guy with the boom stick.
I love shotguns, but 25 yards is getting into rifle territory.