Have you looked into the “comments” capability? That would seem to fit your intent better than attempting to modify normal text to function in an unexpected manner.
From LibreOffice help:"Inserting comments - In Writer, the command Insert - Comment or the Ctrl+Alt+C key combination inserts a comment anchor at the current cursor position. A comment box is shown at the page margin, where you can enter the text of your comment. A line connects anchor and comment box. If a text range is selected, the comment is attached to the text range.
In Calc, Draw, and Impress, the command Insert - Comment inserts a comment.
The author name and the date and time of creating this comment is shown at the bottom of the comment box.
The comments by different authors get different colors. Choose Tools - Options - LibreOffice - User Data to enter your name so that it can show up as the comment author."
I searched on the word “hidden” using LibreOffice help. Several hits were achieved. You may want to look into it.