The current situation is that Spine relies on some other technologies to do what it does. One place where this is true is in the packed textures/atlases, where it relies on the libGDX atlas format which (from some light Googling) only fully supports rectangular atlas/texture regions. The Spine JSON data keeps no information about the textures. It only keeps info about what its name is in the atlas, and how big it should be drawn relative to other images. For more info about what Spine actually exports, read this: http://esotericsoftware.com/spine-json-format/
I can't speak on Esoteric's behalf in terms of how this will change in the future. Nate did hint and some functionality like this, I think, also related to its necessity when implementing one exciting new feature: (https://trello.com/c/fBmxZJ7v).
But looking at the rest of their checklist/Trello boards (https://trello.com/b/frGlgsF7/spine-editor), there are several other features that were promised in the Kickstarter and need to be delivered first.
Also note in the presentation that the new Unity 2D tools specifically does the calculations in savings of fillrate vs mesh complexity. There is always this point where the extra points and irregular shape of the polygon slows down the game instead of speeding it up. If anyone's going to implement this, this is something to keep in mind and I'm not sure how much an external tool like Spine can predict this in behalf of a separate framework like Unity or libGDX or XNA or whatever. Especially for Unity, vertices have some extra overhead to update and push to the GPU that we're not entirely sure of, and the dynamic batching has a vertex limit.
Anyway, just as a heads up, the Spine JSON does not give you any of the info you need to implement this. The atlas may give you a head start, but you'll inevitably have to calculate those meshes based on the alpha data from the image files themselves. And then modify the necessary code in the runtime you're using.
In any case, this type of optimization pushes the envelope for sure. But I've tested Spine's performance in a Unity Project on an iPad1 and it seems to perform just fine with its currently-rectangular parts drawing.