Doc quote ...
NSValueTransformer
is an abstract class that is used by the Cocoa Bindings technology to transform values from one representation to another.
... it's a very simple class, which transforms values between different representations. Sometimes I do use it with Cocoa Bindings, but I heavily do use it with CoreData where I need better (smaller, faster, ...) representation for underlying sqlite store. So, this class has the value for us even if you can't achieve your goal with it.
Also I would solve like you do. New property along with ...
+ (NSSet *)keyPathsForValuesAffectingEmployeeNameForDisplay:(NSString *)key {
return [NSSet setWithArray:@[ @"employeeName", @"employeeDepartment" ]];
}