Your/our problem with local magnetic deviations and the O-GPS1 compass' sensitivity to that gave me a silly idea - much like the ship compass I showed above:
Why not make a sliding device with slightly magnetic objects to the left and right of the O-GPS1? I dont know how well it will work in practise, but here's some kind of proof-of-concept:
I simply placed my camera+O-GPS1 on my desk, pointing due south (180 degrees as the electronic compass would have it) and then approached the O-GPS1 with a screwdriver from the right and from the left:
Moving from afar to touching the hindmost right and left sides with the screwdriver, I got some interesting results:
Click on image to see small video on my homepage.
As can be seen in the video, I could actually enforce from 0 to about +10 degrees deviations at varying distances from the right side and about from 0 to -20 degrees from the left side.
Hmmm......Will it work in practice or not? I don't know, but I do think i will pursue this possibility further!