Yes, I guess that COULD happen, but have never heard of it in real life. Still. it's worth investigating further for safety's sake that's for sure!
As for stacked "full-caliber round balls" such as seen in a .410-bore, that couldn't be done with the 12-gauge in most guns chambered for a 12. I figure -- off the top of my head, nothing scientific -- that a minimal 12-gauge "full-bore size" ball would be something like the .690" (musket-ball) some people (like me) use as a close-up 12 gauge slug. A .690 ball is almost 1oz. right on, I mean about 437 grains. TWO of those would be 2oz. or 874 grains. Three would be impossible in a 12, but not in a .410-bore as you said.
Besides two .690 "full caliber bore balls" not even physically fitting in any 12-gauge shotcup -- that I know of -- I don't think there is even any load data for 2oz. loads...unless you are using 3.5" shells that is...and the gun in question would need to have a 3.5" chamber of course. Mine are 3" chambers, so this would be impossible to do in my two shotguns.
I suppose you could forget the "modern" shotcups and load them the old way -- fiber wads, over-powder-cards, over-shot-cards, etc., if that would make more room in the case for TWO "full-caliber bore balls" of .690 caliber -- but I never learned how to use those components since when I started doing this, the "modern" shotcups had arrived on the scene.
Besides, Maser just wanted to recycle his .451 balls in his 12 gauge, not do anything more, so his buying other larger-sized ball (.690) or slug moulds is not likely.
Well, if worse comes to worst, he won't be able to use his .451 mould for anything except fishing line split-shot sinkers -- or possibly some "gallery-loads" in a .45ACP revolver (which he probably doesn't have anyway) -- but I hope his researching the "jammed balls blow up shotgun and shooter" issue will turn up something more positive for him.
SAFETY certainly is the main point.
-- John D.