Battleships missions are simply changing the lines in the shiparch from TYPE = CAPITAL to TYPE=FIGHTER.
Then you can elaborate somewhat. Personally - i make "clones" of the ships, so copy the entries (not the [simple parts, just the main ship part and collision boxes) and paste to bottom of file. Then rename the nickname to something new - like i used te_ku_gunboat or something. Change the type round, gave them a loadout in the loadouts.ini file pointing towards my new "fighter", then added it to NPCships.ini file - so that the ship and loadout were both for level 19 ships, removed the level 19 encounters parameters from the systems (ie - around planet malta or cretes docking ring) - made them into level 18 - so that the capships weren't appearing all over the place (otherwise they will!), and ensured that the link from npcships.ini to faction_prop.ini was made so they actually use the ships. Then sit back, go find an extremely high paying mission, and have fun.
That guy who made the script is a little late - already done that MONTHS before - with the admins mod, where they choose what the heck they want for players
As for the missions - think it was Michael Dan who worked that one out - about 13 months ago!!