(Speaking unofficially) This isn't part of Spine. It'd be an interesting feature though.
But this does require certain flexibility/functionality on the game engine/runtime side.
But if it's for a runtime that supports meshes and manual UV mapping, and Spine draws parts by generating meshes and assigning UV coordinates, it could theoretically be done by moving the vertices AND the UV coordinates at the same time (I could be wrong though. I'm not too familiar with how UV coordinate data gets pushed to the GPU.).
The other way to achieve the health bar functionality is to use a cutoff shader and an additional texture that determines the mask and it'd have nothing to do with Spine. But it would give you more options for the shape of this bar. This would also depend on the flexibility of your engine.
But right now, judging from the Trello boards and kickstarter feature list, it's neither an existing nor a planned feature.