I know a number of people have asked the same question here and there, though I like that you point out the feature clearly.
That said, this isn't something most 3D engines are built for (which are built around the language of triangles and polygons and common graphics library features) so it's far from something standard.
But as the next generation of games comes along, and hand-drawn-and-painted-2D games are becoming more common and devices become more powerful, I think support for it is an interesting question, even if the feature it's far, far, FAR into the future.
You should know that it's not something that most engines with limited APIs can support. I think that's something that factors a lot into what can and can't go into Spine.
All I can do is guess though. I'm not part of Esoteric. :p