Hi,
Here is a workaround of doing this.
Although you cannot tell TypeMock that the call to MyBaseClass will be mocked. You can tell TypeMock that the call to the first constuctor will run as normal and the second constructor mocked
[Test]
public void TestBase()
{
MockManager.Init();
Mock mock = MockManager.Mock(typeof(MyDerivedClass),Constructor.NotMocked);
mock.ExpectUnmockedConstructor(); // call to MyDerived is made
mock.ExpectConstructor(); // call MyBase - mocked
MyDerivedClass d = new MyDerivedClass();
MockManager.Verify();
}