Combat performance in tough situations is about keeping weapons and engines online for the next seconds or minutes, I have a hard time seeing how crew quarters can be vital in that situation.
Well if your ship loses its crew quarters the best you managed is called a draw, and if your opponent spent less to kill your crew quarters than you spent to wreck his guns, it is actually a win for him. your proposed idea of putting crew quarters outside the armor belt is something that no one has ever done with a battleship before.
If you want HTK to change(what magazines and turrets call armor is really just additional HTK, these are two different concepts and they are handled differently), then crew quarters are already minimally protected. any less and all of it would evaporate on the 1st hit. Likewise, the max a railgun can have is larger than the max a magazine can have, and is only surpassed by reactors and turrets, and in the case of turrets consider that a quad turret has the HTK of 4 lasers/mesons/gauss cannons before "armor" is added.
That said, post your battleship design, how much tonnage you don't want protected by the belt, and I'll do the math for what the armor would cost without various components being added to it, so you can see how better protected your guns would be under your model.
I do personally think that turrets should be outside the belt, and that their armor cost should be cut by 1/4th, to make that more useful to people to armor turrets.