One thing which puzzles me is the general lack of other Constitution refits out there.
I think it's just that they (the studio) reserved the hero ship for 1701, that's all. And when they designed it, I don't think they went "oh, we should make it so that it's physicall possible in-universe to have this upgrade." They just designed something that looked the way they wanted.
I mean, if they had kept Ken Adam's design for Planet of the Titans, for the same reasons, is there a way we could fit that with the original design in-universe ?