If there are enough clouds in the sky, you should be able to use mocha to track. One technique is to “unlink” the layer, so it does not move. As the camera pans and clouds go off-screen, the unlink planar track will create a track offset.
An alternate approach might be to track a static foreground element with mocha AE or Pro v3. Do a camera solve and then push the solved track in z space to approximate here the sky plane is.
Best of luck,
Ross
Ross Shain
Imagineer Systems
http://www.imagineersystems.com