I solved it.
But it's complicated... so I'll describe only generally:
I create a polygon (in graphic layer attached to ground) whose points are identical to the area measured. I label the polygon with the relevant info, then I erase the view model.
My customized "measurements-wizard" widget can display concurrently many measurements ot 3 types: distance (with slope), area, and volume (which is "extended" area neasurement).
The user can replace the orange/white chess board of an area or volume measurement by bare polygons, (whose internals are transparent) labeled with the relevant info.