Our party has a light ballista, so for grins and giggles I'm going to take leadership to get a Siege Mage cohort. I'm looking at ways to reload the ballista. Researching a custom spell may be possible (the DM ruled that Invisible Servant and Reloading Hands would not work for a ballista).
One way would be to build a construct or two. A large animated object could have hardness 10 and would be able to reload it in one full-round action. Would be buildable right away. Even with hardness, it could eat an AoE and die.
Junk golems also have promise. Couldn't build them until the cohort is 7, but they have magic immunity so random AoEs are not a concern. Building a pair would let them give each other fast healing 2 (even though they would occasionally take 1 swarm damage) to top off after a battle. Two of them could keep the ballista up every round. Would be more vulnerable to weapon damage with DR 5/adamantine.
