One idea that came to my mind. Since it's a bit hard for me tell right away what is exactly necessary, and therefore I can't really make a concise request, perhaps a better model would be something like this:
basically you "join" the project and get to know it a little first. Then if we identify some concrete area/task that requires a 3d artist I just commission you to do that. And so it's like a continuous loop of collaboration. I guess that would make more sense.