From the C# Language Specification 5.0:
The set of values that an enum type can take on is not limited by its
enum members. In particular, any value of the underlying type of an
enum can be cast to the enum type and is a distinct valid value of
that enum type.
So this is completely alright to do and would be your best bet:
throw new WebFaultException((System.Net.HttpStatusCode)429);