Dont't model the catapult as one MDL. Rather, use a series of MDLs put together to model the catapult.

For example, use Three Models to build your basket. Now, put these models (which could be as simple as just rectangles) on the end of you catapult arm and rest your Projectile within this basket. Now it shouldn't roll away and as you move your Catapult arm, it should carry the projectile with it and simulate a Catapult.

Since you don't have the option of concave geometry till octree comes online, this is the only way (I think) that you'll get the basket effect neccessary for a catapult.