type
  TMyHintWindow = class(THintWindow)
  protected
    procedure Paint; override;
  end;

procedure TMyHintWindow.Paint;
begin
  // hier HintWindow-Inhalt zeichnen
end;

procedure TDemoForm.ApplicationShowHint (var HintStr: String;
  var CanShow: Boolean; var HintInfo: THintInfo);
begin
  if (HintInfo.HintControl = TreeView1) then // hier Control einsetzen
    HintInfo.HintWindowClass := TMyHintWindow;
end;

procedure TDemoForm.FormCreate (Sender: TObject);
begin
  Application.OnShowHint := ApplicationShowHint;
end;