Since a user type can be(in this case is) composed of multiple columns, one would have to be able to actually select/reference a specific column, otherwise the cast or actually any other operation in case of multiple columns doesn't really make any sense. A single column user type should actually be no problem.
I think the actual problem is the Columns annotation which makes it impossible to refer to the respective columns. Some kind of model, like an embeddable would actually be needed.