After some trial-and-error, I finally figured out how to render skybox faces using the camera animation keys in Terragen 3 (Pro w/Animation).
Initially I tried simply setting 5 keys (one for each face, ignoring down). Everything rendered fine when jumping to a specific keyframe and rendering manually, but as soon as I rendered a sequence, the camera angles were off and the images wouldn't line up.
To get it working, I had to set 3 camera rotation keys per box face, and then use the middle one when rendering the sequence.
My camera rotation keyframes are:
1,2,3: x=0, y=0, z=0 (front)
4,5,6: x=0, y=90, z=0 (left)
7,8,9: x=0, y=180, z=0 (back)
10,11,12: x=0, y=270, z=0 (right)
13,14,15: x=90, y=0, z=0 (up)
Then when rendering the sequence, I start at frame 2 and step by 3. Or when rendering via pixelplow, render frames: 2,5,8,11,14.
Is there an easier way to do this? I'm new to Terragen 3, so maybe I'm missing an option somewhere?