A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
You're correct, only placeholders on the slide layout can be modified on a slide created from it. Graphic objects on a layout (like lines) will always be ineditable on the slide.
As a workaround, create a sample slide with a single-cell table. Format the left edge of the cell with a line. The line will automatically grow and shrink as the text inside the cell is edited. Users will have to copy and paste the sample slide to maintain this effect.