At present it isn't possible to create a watermark with text on a sheet so that it is behind the drawing itself. That is, to allow the linework of the drawing to write over the top of the words of the watermark. Text wins! Drawing loses!
Here's one way you can...yes, work around warning!
Create a text string in AutoCAD...yes I said AutoCAD...at the desired plotted size in model space. Then import and link this string into a Legend View set to a scale of 1:1. Change the dwg setting to Background.
Now you can place this legend view on any sheet you need it on and it will be masked by the model and annotation but still be present. Since it is linked any update to the dwg file will show up in the legend view as well. Here's the result as well as examples of some alternatives that don't in view.