typedef void __fastcall (__closure *TMyIBTableOnGetExcelString)(AnsiString &Text, TMyIBTableRec *rec, AnsiString FieldName);

class PACKAGE TMyIBTable : public TComponent
{
private:
  TMyIBTableOnGetExcelString FOnGetExcelString;
  void __fastcall SetOnGetExcelString(TMyIBTableOnGetExcelString value);
  TMyIBTableOnGetExcelString __fastcall GetOnGetExcelString();
public:
  __property TMyIBTableOnGetExcelString OnGetExcelString  = { read=GetOnGetExcelString, write=SetOnGetExcelString };
};


void __fastcall TMyIBTable::SetOnGetExcelString(TMyIBTableOnGetExcelString value)
{
  if(FOnGetExcelString != value) {
    FOnGetExcelString = value;
  }
}
TMyIBTableOnGetExcelString __fastcall TMyIBTable::GetOnGetExcelString()
{
  return FOnGetExcelString;
}



// и в нужном месте:
void TMyIBTable::somemethod()
{
    ..............
        if (FOnGetExcelString) FOnGetExcelString(Text,rec,fn);
    .............
}