Just found this out and thought it could be useful to other modders. Ships can rank up into themselves which, combined with rankup[resource] = x and nextrankxp = 1, can be used to let a ship pillage resources every time it gets a kill. I had assumed sequential xplevel values were necessary, but apparently not.
This would be excellent for an Orion Pirate faction. Perhaps their ships would not gain any points when ranking up, but would give the player resources every time they destroyed another ship. This would need to be such that the ships one could build with those resources would equal the amount of combat power that the other factions gain through rank-ups.