This feature is really important to us. I am wondering as if for a first step in making this work it would help to have the ability to mark entities to be exported as either a class or trait. Even if it wasn't possible at first to link the traits to other entities within Skipper, at least it would be possible to design, maintain and export them. They would linger in the region independently at first. Since they're easily added within the class by using "use TimestampableTrait;" which is code Skipper doesn't replace there would be a basis for being able to use them.
Even better would be if there was a "Trait" tab (maybe you could integrate it into Inheritance), all one would really need to do is assign the trait to an entity, there are no other options required and the export would really just result into the "use" line being added to the class. This way it could be visually integrated into the map as well.
The problem right now is, if one uses Skipper to define the traits, exports the classes, changes the class to trait and re-exports, Skipper will convert it back to a class plus keep all the trait stuff around as junk. So my proposal for the first step to at least being able to define export as trait would at least help keeping everything in sync.