i know it doesn t fulfil your requirements for a battleship, scorn - but when looked upon it objectivly, - can t one say that the liners are some sort of "civilian battleships"?
they have the size and .. in a way, the weaponry. 12 turrets is cruiserlike, allright. when it comes to a "true" civilian ship. - i wouldn t see much more than transport turrets on it. - with the new lvl4 transport turrets, those ships are basicly fortresses a sole pirate will rathe avoid, cause - there are simply easier targets around than liners.
of course those ships do not have the armor or the shields of a battleship. - so, the question is. if there was a ship of roughly the size of a battleship ( i do see civilian ships to be bigger and - sort of less efficient when it comes to pure combat - but most certainly putting more importance on comfort ), with like 16 turrets of limited firing arc ( max lvl7 freighter turrets ), 250.000 shields and around 350.000 armor ( normal BB shields, low BB armor ) - would that count as a civilian battleship?
and how useful would something like that be. - its basicly a liner with the protection of a smaller battleship and a size that makes it as hard to fly as a spyglass. ( thats how i d see something of battleship class for civilians )
personally, i do not see pure civlians to build up a ship that can support military BB turrets, cause there is no need for it. transport turrets can fend off pirates just fine and are - as a whole, a lot more efficient, when it comes to the price / performance ratio. stripping a shield on a civilian ship shouldn t be an issue from a roleplaying point of view - aswell as armor.
edit: about the rarity of such a vessel - something of bretonian royal liner rarity feels right. - if its more of a roleplay ship than a pvp ship, it should not be spammed all over sirius, especially, when its expensive.