NHibernate mapping with a class hierarchy whose base class is abstract and the discriminator is not a string

public abstract class BaseClass
{
...
}

public class ChildClass : BaseClass
{
...
}

DiscriminateSubClassesOnColumn<byte>("Type")
    .SubClass<ChildClass>()
    .IsIdentifiedBy((byte)OperationType.Plan)
    .MapSubClassColumns(p => { ... })

SetAttribute("discriminator-value", "-1");

DiscriminateSubClassesOnColumn<sbyte>("Type", (sbyte)-1)