The pirate "ships" are mainly speedboats that they use to get boarding parties onto the ships. One 250-lbs JDAM would turn one into splinters
The Predator B (the new, larger version of the old Predator) can carry 3,000 pounds of ordinance, plus 3,000 pounds of fuel, giving it a loiter time of 32 hours at 50K feet altitude. I would guess you could put a pair of 1300 lbs harpoons on one and stay within the weight limits, but I'm thinking you would need to redesign the hardpoints on the wing