In the v 1.41 6/6/2006 release the “Search for a Class Type” window in Object Mapper no longer locates enums within a business object. You can still type in the enum name in the Custom Data Type field and everything works okay.
-Larry
I just tried compiling a project with the 'extra' designer file and it creates a bunch of compiler errors. Just leaving the nested class un-configured sounds like the easiest solution.