Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

type
   TSaveRedir = packed record
     Addr: Pointer;
     Bytes: array[0..4] of Byte;
   end;
   PSaveRedir = ^TSaveRedir;

 procedure RedirectCall(FromAddr, ToAddr: Pointer; SaveRedir: PSaveRedir);
 var
   OldProtect: Cardinal;
   NewCode: packed record
     JMP: Byte;
     Distance: Integer;
   end;
 begin
   if not VirtualProtect(FromAddr, 5, PAGE_EXECUTE_READWRITE, OldProtect) then
     RaiseLastWin32Error;
   if Assigned(SaveRedir) then
   begin
     SaveRedir^.Addr := FromAddr;
     Move(FromAddr^, SaveRedir^.Bytes, 5);
   end;
   NewCode.JMP := $E9;
   NewCode.Distance := PChar(ToAddr) - PChar(FromAddr) - 5;
   Move(NewCode, FromAddr^, 5);
   if not VirtualProtect(FromAddr, 5, OldProtect, OldProtect) then
     RaiseLastWin32Error;
 end;

 procedure UndoRedirectCall(const SaveRedir: TSaveRedir);
 var
   OldProtect: Cardinal;
 begin
   if not VirtualProtect(SaveRedir.Addr, 5, PAGE_EXECUTE_READWRITE, OldProtect) then
     RaiseLastWin32Error;
   Move(SaveRedir.Bytes, SaveRedir.Addr^, 5);
   if not VirtualProtect(SaveRedir.Addr, 5, OldProtect, OldProtect) then
     RaiseLastWin32Error;
 end;


 // Example: Replace Application.MessageBox with your own. 

function MyNewMessageBox(Self: TApplication; const Text, Caption: PChar;
   Flags: Longint): Integer;
 begin
   ShowMessage('New Messagebox');
   //.... 
end;

 procedure TForm1.Button1Click(Sender: TObject);
 begin
   Application.MessageBox('You'll never see this Text /
     Diesen Text wirst du nie sehen'', '...', MB_OK);
 end;

 var
   S: TSaveRedir;

 initialization
   RedirectCall(@TApplication.MessageBox, @MyNewMessageBox, @S);

 finalization
   UndoRedirectCall(S);