Loupe Overlay is useful for image layouts in Loupe View (Magazine covers with text, etc) however the Overlay is not visible in Full-Screen mode [F].
If this were possible, images could be shown, or projected, 'full-screen' with an overlay similar to a watermark or Identity Plate.
Regards. My System: Lightroom-Classic 14.0, Photoshop 26.0, ACR 17.0, Lightroom 8.0, Lr-iOS 9.0.1, Bridge 15.0.0, Windows-11.