решил:
type
Visit = procedure(x: String);

Procedure TraverseStack(S:stack; NewVisit: Visit);

Вызов
interface
...
procedure MyVisit(x: String); {работает без объявления???}
...
implementation
...
var
...
MyNewVisit: Visit;

{$R *.dfm}

{$F+}
procedure MyVisit(x: String);
begin
ShowMessage(x);
end;
{$F-}

procedure TForm1.FormCreate(Sender: TObject);
begin
CreateStack(MyStack);
MyNewVisit:=MyVisit;
end;

procedure TForm1.Button6Click(Sender: TObject);
var
mystr: string;
begin
mystr:='good';
TraverseStack(MyStack, MyNewVisit);
end;