type
 TPanel = class(ExtCtrls.TPanel)
 private
   FOnPaint: TNotifyEvent;
 protected
   procedure Paint; override;
 public
   property Canvas;
   property OnPaint: TNotifyEvent read FOnPaint write FOnPaint;
 end;

 TForm1 = class(TForm)
   procedure FormCreate(Sender: TObject);
   procedure PanelPaint(Sender: TObject);
 end;

...

procedure TPanel.Paint;
begin
 inherited;
 if Assigned(OnPaint) then OnPaint(Self);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Panel1.OnPaint := PanelPaint;
end;

procedure TForm1.PanelPaint(Sender: TObject);
begin
Panel1.Canvas.TextOut(100, 100, 'тест');
end;