With the image elements, you can get the canvas position of the corners to help work out where line start and end points are: https://developer.playcanvas.com/api/pc.ElementComponent.html#canvasCorners
Alternatively, you can try to use an image element for the line and position it via the global/world position using the information here: Dropdown For Different Aspect Ratios - #4 by yaustar
It’s going to be a little tricky no matter which method you use though