Well, it could be done but alot of your ideas for it aren't easily explainable or easily done by a large amount of people, because if you want to be able to choose the ammo and the target that means that you're going to have to animate each ammo type landing onto each target type, and vice versa. Making it hard to divide up the work between animators, and on top of that you're going to have to find someone who will code it all for you. You're better off just animating it all yourself. When you're trying to make a successful collab, you want an idea that original, relatively easy to explain, easy to divide into groups with short parts each, [ex:animating each room of a castle for a castle invasion collab] and it has to appeal to the masses in terms of entertainment AND animating capability [can be easily animated by noobs and pros].
The catapult collab has some, but not all of these qualities, and the ones its missing are vital. I just don't see it working, unless you can devise some way to split the work up between animators easily without any confusion and find a coder who is willing to code it for you.
Keep in mind I'm not stopping you, this is just advice from a collab vet.