They would entail converting rectangles into diced meshes for RegionAttachments, and MeshAttachments would just plain not work because the UVs would be all over the place. We currently have no plans of supporting using diced sprites.
If you are using Spine Pro, one thing you could manually do to optimize texture space (applicable to any runtime), is have a hand-packed atlas image where you minimized the empty areas already instead of having all the individual images, and then in Spine, map to different parts of that one hand-packed atlas image using meshes.
How much effort that involves and how much the effort is worth depends on a lot of factors, I guess.