Duplicate the view (most important part!). Trace around the required area with your filled region. Go to view properties, set "display model" as none - then set the view to wireframe mode.
On the sheet, overlay the filled region view (the duplicated one) ontop of the view you duplicated - Revit will align it correctly for you, and it will appear as transparent
as Jean d'arc found out - it wont work with a solid hatch, try a very tight crosshatch to get the same effect.