[GraemeJ]

I think the main point is, there are several different options that enable you to achieve what you want without further complicating the program - a complication that I doubt would be required or used by many.

Here's anther idea for you, based on what has gone before and assuming you use a pedal to turn pages.

Make up a pdf page with whatever comments you want on it. Attach it to the end of the chart the comments refer to. In Settings, check 'Prevent Pedal From Changing Songs'. Now, you get to the last page (the one with your comments on) but can only move to the next chart by physically tapping the screen.

No placeholders and your comments are permanently attached to the chart involved.