Oshyan, thank you for your reply.
Yes, I can animate the plane to the shape of the clouds, that's no problem. I just need some visual cue to tell me where the clouds are.
Is there a way to place some objects, such as boxes, in Terragen 4 to "block out" where the cloud is, then export these with the camera to Maya? I could then use the boxes to help me animate the path of the plane. Then I would delete the boxes, as they are only placeholders. It's a clumsy method, but better than nothing. I really want to take advantage of the fully 3D physical clouds in Terragen, and fly through them as you would fly through a canyon. Do you think this is a feasible way? I understand your method using a depth pass and doing it more in the compositing stage, but I really want to do it in 3D, and affect the flight path of the airplane based on the position of the clouds.
Thanks again for your expertise.