This is actually more like a libGDX atlas limitation than a Spine limitation.
But if you had a packer and atlas format that could do this, Spine still wouldn't be able to use it.
Unity Pro does this for its new 2D sprite system (which isn't so great for skeletal animation, by the way. But very nice for building environments).
Also, wow. This example is exaggerated. There would need to be some padding between hulls or images would bleed into each other when UV-mapped onto meshes. Either that or meshes would be incredibly wasteful in terms of number of vertices. The example seems to be for a "pure" 2D-rendered game.
There's also quite some work to be done for Spine to be able to interpret this type of packing, if such an open format existed.
