I felt like this could be pretty useful in Space Escape, because where your ship aims is not necessarily where you are going, and having your bearings relative to the Earth is pretty important when one is setting out to do a gravity assist maneuver.
After some work, I turned up with this:
|Degrees show exactly what it's for, and it follows the Earth as you move about.|
I'm not familiar with how Quaternion rotations work, and I'm still wrapping my head around some of Unity's rotation math. Getting the right kind of code took maybe an hour of actual work, but this is the behavior that drives the... behavior:
All of this is viewable from the web player here.