I had to solve this exact problem for a client not long ago. My solution was to simply create two markers; one at each LatLng for the overlay's LatLngBounds that, when moved, would update the overlay and log the LatLndBounds to the console for me to reference. I also added reticules (vertical and horizontal lines) that are shown when dragging a marker so it was easier to visually position the overlay.
I cannot share the code from the solution with you, but it involves working with an OverlayView in order to convert LatLng coordinates to and from pixels via MapCanvasProjection.
Like you, we didn't need this functionality for end-users so I added a "authoring mode" that's toggled by providing a debug parameter to the solution's query string.
