Ok, I've got the switching between scale-from-center or scale-from-corner working.

The way it works is like this - when you move over a corner grip, an origin point will then pop up showing you where the scale will be centered around if you drag:



If you click the corner handle instead of dragging it, it will flip the scaling origin from the center to the opposite corner:



Each click (instead of drag) on a corner handle will flip it between those 2 scaling styles.

- Michael