Rhino Mocks: How can I mock out a method that transforms its input?

Expect.Call(delegate {dao.Save(transaction);}).Override(x => x.IsSaved=true);

public delegate void FakeSave(Transaction t);
...
Expect.Call(delegate {dao.Save(t); }).Do( new FakeSave(delegate(Transaction t2) { t.IsSaved = true; }));

Expect.Call(() => dao.Save(transaction))
    .Do(new Action<Transaction>(x => x.IsSaved = true));

Expect.Call(delegate {dao.Save(transaction);})
    .Do(x => x.IsSaved = true);

ITransaction transaction = _Mocker.dynamicMock<ITransaction>;
Expect.Call(transaction.IsSaved).IgnoreArguments.Return(true);
_mocker.ReplayAll();
dao.Save(transaction);