A couple of suggestions...
No, I don't have a Judge, and don't plan on getting one. But I do have some experience with the .410 shell out of Contender.
You ought to see if you can get some .410 slugs! OK, they are going to be undersize for the bore, but they are pretty snappy from the Contender, and I think might dump more energy than standard .45 Colt loadings. The box says 1600+fps for the .410 slug, but I'm sure thats from a regular length shotgun barrel. Still it ought to be doing pretty well from the short pistol, probably ahead of the .45 Colt in the Judge?
Second, consider the 2.5" shells. They seem to pattern better out of my Contenders, and while I have not shot any buckshot I don't see you loosing much over the 3", except maybe a single pellet. Balanced against the possibility of the opened shell (3") tying up the gun, the 2.5" seems to be a better choice.
Personally, I think the Judge is all marketing hype. You won't get any great amount of pattern dispersal at across the room (or less) personal defense range, so if you are not on target, buckshot isn't going to help you any. I think a .45 Colt (only) revolver would be a better choice, but that's just my opinion.
Good Luck!