[mike golding]

The indicator would serve as a memory aid when playing either the left or right side page. With alternate page turning you can find and tap the pedal wherever on the page is most convenient. Playing the left page you see that you need a page turn; on the right page you see that you DON'T need a page turn.

So here's an alternative idea that seems simpler to implement than wrangling the vertical bar (empty space) width. When "out of sequence" just put one or more vertically arranged small left arrows inside the bar (white on black?). Kind of subtle?