Don't have much use of this, but I can try to implement this.
Can you provide me with some example grids you would like
to have.
One problem that may arise is, I don't know if the aspect ratio
and field of view of the viewfinder image is always the same as for
the resulting image.

Would you like to test this for me, capture an image and
make a screenshot of the viewfinder image and compare both.

Nicolai