Here's another idea:

Camera-button: drag-modifier

It would be really useful to use the camera button, as a modifier for dragging the page. When the button is down, any pen stroke will act as a drag, rather than a write. When the button is up, the pen will write as normal.

This will make panning around the image much faster and more pleasurable if you're holding the phone and jotting notes!

}:^)~