[__cadetill]

Si quieres mostrar tu propio diálogo

  private
    FAbout : string;
  published
    property about: string read FAbout write FAbout;

  TAboutProperty = class(TPropertyEditor)
  public
    procedure Edit; override;
    function GetAttributes : TPropertyAttributes; override;
    function GetValue : string; override;
  end;

implementation

function TAboutProperty.GetValue: string;
begin
  Result := Format('(%s)',[GetPropType^.Name]);
end;

function TAboutProperty.GetAttributes: TPropertyAttributes;
begin
  Result := [paDialog];
end;

procedure TAboutProperty.Edit;
begin
  inherited;

  AboutBox := TAboutBox.Create(Application);
  try
    AboutBox.ShowModal;
  except
    on E: Exception do
      raise ECreateForm.Create(ErrorCreateForm + E.Message);
  end;
end;

uses en D6 o superior : DesignIntf, DesignEditors
uses en D5 (o inferior creo) : DsgnIntf

Si no me he dejado nada, esto es todo

Espero te sirva