This is an interesting question and one I have thought about. There is no layer in Designer like there is in something like Illustrator. PDFs do support layers but there is no way that I am aware of to achieve this with Designer. You could possibly fake it by having a margin on the right or left and place a field on the Master page. You would then need to establish where the cursor is on the page and reset the fields coordinates.
Not sure if this helps. As I said interesting but I have not attempted it.