Conditional mapping of entity id using mapping by code and nhibernate without fluent nhibernate

Question

I'm trying to convert from fluent nhibernate to the build in mapping by code (Loquacious) in 3.2 now using 3.3 of nhibernate.

The problem I have is conditional mapping of the Id column depending on type.

I'm now using something similar to this which assumes the entity id is of type guid.

var mapper = new ConventionModelMapper ( );
mapper.BeforeMapClass += ( modelInspector, type, map ) =>
{
    map.Id ( k =>
    {
       k.Generator ( Generators.GuidComb );
       k.Column ( type.Name + "Id" );
    } );
};

How can I configure/setup for when an entity is using a string (default as assigned)?

Do I have to override with custom mappings for all entities not using guid as an id?