My be there is a different method.
Let's suppose we know the magnetic (or true) course between two points on the earth clearly visible from the air, lets say two towers, couple of miles apart. We could position our aircraft's axis during flight to the extended line connecting this two towers. A short glace at the compass in the right moment when everything is in row will give us the reading of the compass with known course.
If we can do this for more headings, we will have at least the readings of the compass for known heading, even if not for the right 360/90/180/270 headings. Unless we find the right towers of course
The rest is a little interpolation.
Miroc