But Spine editor doesn't even support addons.
I think you might have misunderstood Shiu. He was talking about flipping in Spine Editor, the program you use for animating.
In terms of interfacing UnityEngine's animation system with the SkeletonAnimation and Spine.Skeleton objects.
It seemed pretty obvious to me that the answer would be to set up an interfacing class between Unity's animation system (which can only invoke public methods on attached MonoBehaviours) and Spine, but I've gotten mixed results, mostly because of Unity doing unpredictable stuff when you invoke methods through Unity's animation event triggering. I haven't tried to do this again since 4.5 though.
Regardless, Spine itself has no trouble responding to method calls 'cause that's just how it normally works in all other uses. It just works.
Anyway, ideally, the interfacing class would know how to change flipX flipY on Spine.Skeleton, and maybe do more complicated things like tell the skeleton to switch attachments.