Custom Identity Types in Mapping

I have an application where the identity column is stored as an Oracle VARCHAR2(50 BYTE) but is actually a Guid. I want my model to expose it as a Guid:

clas开发者_开发技巧s Foo
{
     public Guid Id { get; set; }
}

Using Fluent NHibernate I don't see a CustomTypeIs() method on the IIdentityPart. I would think it would be something similar to an IUserType, but I can't find the correlation. Any thoughts?

Fabio Maulo noted that this is governed by the DataProvider on the NHibernate Users mailing list.