But I doubt they will ever have the guts to flat outlaw 12 ga shotguns. - Bullpup 'style' sure, they will no doubt try.
They already have. But regulations require enabling legislation; there is no enabling legislation that would allow them to ban a double-barrel or semi-auto bull-pup shotgun whose bore size was 0.499".
Actually, there are already a variety of interesting non-class-III guns available that shoot .410 shotshells. For example, I've seen a revolver which holds five .410" shotshells and has probably about a 3" rifled barrel; the rifling is not for accuracy, but rather to make the thing be a "handgun" rather than a "shotgun". My suggestion to use .28ga, possibly in a longer cartridge, is intended to produce something more powerful than a .410 in "interesting" configurations.