When the domain class was split into triggerHandler and domain classes, the PR #300 author @wimvelzeboer mentioned that domains could now be arbitrary ApexTypes

It would be nice to see this sample repo extended to show such an example. This would include:

• Factory construction in the Application.cls
• ApexMocks support (enabled by Factory construction)

As I understand it, such domains of arbitrary ApexTypes extend fflib_Objects but I believe to implement factories and mocking support, some changes are needed in fflib_Application.cls to support something other than SObjectTypes as keys to the constructor class.