I think that broadside warfare simply isn't very well suited to the gameplay pace that Starsector has been built around. (or put another way, the gameplay pace isn't very conductive to broadsides) Right now, the speed of the combatants and limited engagement time dictated by flux (at least on most combatants, the Onslaught and others are exceptions) means that it would be hard to decisively engage ships of the same size class by presenting a broadside - you need some way to remain close to deliver a finishing blow. Some broadside Harpoon pods might do the trick, but I think that goes against the spirit of what you want - large ships trading gun broadsides. And frankly, I find the current 'skirmishing' gameplay to be extremely fun, even if it isn't the most realistic.
tl;dr I don't think broadsides are a good fit for Starsector up until the capital class, and broadside-type capitals already exist. You could build a great broadside-focused game on Starsector's chassis, perhaps a total conversion mod, but it wouldn't be a good fit for the base game.