I've not tried it, what's HCl like on through hulls/seacocks? A bit splashed on the outside should show if it dissolves the metal, it'll clean it up and make it shine for a while.
I'm a bucket chemist and fairly gung ho but I wouldn't be leaving anything soaking while I was away. Yes, use HCl to dissolve the limescale while you are there to watch it fizzing (producing CO2), but flush it out well before you leave the boat.
You will get chlorine if you get acid on bleach, don't mix your cleaning materials.
Any bull in this post may be composted.