Actually a vertical scroll bar to the extreme left or right edge of the screen that shows movement of the Z would be sufficient .. The bar could be 'scaled' according to the limits of movement established for Z axis on F1 screen. That way it would be an accurate representation on each machine. Perhaps an arrow or some means of marking the point on the bar where the Z work offset has been set, and a moving marker to indicate + & - movements of Z axis.
Yes .. similar in appearance to the scroll bar that's found on the side of most any open program on the computer .. with a darkened moving section that could be scaled in size to the current tool length .. and some kind of fixed marker that identifies the Z work offset location so you have a visual on where the tool is in relation to the work piece.
That sound like a great idea @gmarsh1 . A graph showing the current tool Z depth and a fixed reference to the final depth of cut would be great. On first runs I often find myself wondering how much deeper the toolpath needs to cut and did I leave enough tool sticking out of the collet!