The affect is for the module to fail midway through loading. An exception is thrown by boost python which is then caught later on by boost python. Any classes exposed after exposeDerived() will not appear in the module.
My workaround for this is to not name the python properties the same name.
I rename "Name" for Base::Name to "Base_Name" and "Name" for Derived::Name to "Derived_Name", this seems to fix it. However it might be useful to throw up a more informative message, or to right something to stderr, it was quite difficult to find.
Luke