Ами можеш да ги изкараш да са публични. Дефинирай си един клас наследник на TDBGrid:
type
  TMyCustomGrid = class(TDBGrid)
  public
    function CellRect(ACol, ARow: Longint): TRect;
    property Col;
    property Row;
  end;
{ TMyCustomGrid }
function TMyCustomGrid.CellRect(ACol, ARow: Integer): TRect;
begin
  Result := inherited CellRect(ACol, ARow);
end;
и след това го извикай така:
procedure TForm1.Button1Click(Sender: TObject);
var col, row: Integer;
begin
  col := TMyCustomGrid(DBGrid1).Col;
  row := TMyCustomGrid(DBGrid1).Row;
  ShowMessage(IntToStr(TMyCustomGrid(DBGrid1).CellRect(col, row).Top));
end;
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