In the shown example, it’s the place to the left of where it says “Date� and above the place where it has “2925�. I would like to write into there.

    Oh, that little area is neither cell nor header.. that's a perfect use for CONTEXT_ENDPAGE.
    Here's how to write to that area, shown by drawing an "X":

      case CONTEXT_ENDPAGE: {                   // after page headers/cells are drawn..
        // Draw black 'x' in the dead area
        int X1=x(), Y1=y(), X2=x()+row_header_width(), Y2=y()+col_header_height();
        fl_color(FL_BLACK);
        fl_line(X1,Y1,X2,Y2);
        fl_line(X1,Y2,X2,Y1);
        return;
      }