Set your camera Y position (middle value) very high, -90 for X rotation (first value), and set the camera to Orthographic mode. Then turn off rendering of atmosphere and set the sun to a straight overhead position as well (otherwise you get shadows "baked" into your texture which may look odd if your game engine provides shadowing). Then just render a square image at whatever size you need the texture at.
Unfortunately this is a fairly inefficient approach and requires very large resolution to get good quality. TG2 is not particularly good at high resolution renders at this point, but I don't know the capabilities of your engine or the detail you're trying to reach, so 2000x2000 or so may be ok for your needs, and is very doable in TG2 already.
- Oshyan