Place text on borders of the figure, border of the subplots, on the inner-side or outer-side. Also control the most important text-properties with one simple FormatString.
Text stays on the borders, even while zooming, panning, etc.
Try 'bordertext' without arguments to see the demo (figure is added as screenshot)
Thank you Arnold for your comment (and rating). I've solved the background overlap and I added the matlab syntax.
Your other suggestion to add transparency to the text background is impossible, because the text object has no BackgroundAlpha property.