I wouldn't say the
possibility of other nations developing heavy siege artillery is ahistorical, it's just that since no other nation was contemplating having to assault the Maginot Line, no one else saw the need.
Actually, the US did work on the
Little David mortar late in the war, in anticipation of having to crack extensive bunker complexes in an invasion of Japan. It would probably have fallen somewhere between the German "Karl" mortars and "Dora" railway guns in terms of firepower, though its bore was larger than either.
A mod to simulate such weapons in-game would first require some development path to acquire siege artillery technology. Once available, siege artillery units could be produced. These would be treated as division-sized, reflecting their disproportionate cost and logistical burden, but would have practically no defenses, such that
any damage would kill them, perhaps even in the course of an otherwise successful attack (such guns were subject to extreme wear and tear anyway). An attack by one of these units against a fortified province would do miniscule actual damage, but would trigger an event that would reduce the fortress level in the province by some reasonable amount. As a practical matter, this would probably require that the fortress busters only be useable against specific provinces which were either the site of extensive pre-war fortifications (e.g. the sites of the Maginot Line, Sevastopol, Leningrad, Corregador), or are the likely site of heavy fortification in the course of a game (e.g., Tokyo).
It would thus be very unlikely that a player would bother to develop these units except for the sake of his own megalomania. I'm not sure how prudent an AI would be, though.