Well, I have some bad news after poking around with everything this weekend and trying to see if I could figure out a solution.
SDL unfortunately lacks a critical function that would make it relatively straightforward to duplicate the SFML extension functionality: namely, the ability to render textured polygons. There is actually an SDL library called sdl_gfx that grants that functionality, but unfortunately the function that facilitates that only takes SDL_Surface pointers to represent the texture itself, and as some of you may know with SDL2, there is now a hardware-accelerated version of surfaces called "textures" that is sort of replacing surfaces. So in theory, one could use the sdl_gfx library to accomplish a Spine SDL extension, but only by using the old-school surface blitting rendering method. For myself, I have gone the way of hardware accelerated textures in my game engine, which would mean that I would have to rewrite my rendering itself in order to build the Spine extension, and in so doing I would be essentially reverting to a less efficient rendering method.
SDL2 does allow for rendering rotated textures, so if rotational/anchor point data was used for the textures displayed at sockets, it could be done. Conversely, I'm sure someone who's better at math than I am could probably write some code to covert the skeleton vertexes to rotation angle/anchor points, although I'd guess that would be a mighty bottle necking calculation.
Long story short, I just don't think I'm going to be able to pull it off given the current state of SDL2 and the SDL2_gfx library. If the guy who's responsible for maintaining SDL2_gfx someday switches things up to use textures as opposed to surfaces, then I would have the ability and interest in doing it (I suppose I could just fork his code to make that happen, but I suspect that again there would be some math involved there that would be a bit tricky). So while in theory it might be possible, I think it's a bit above my skill level, and instead I think I'll have to look at other solutions for 2d skeletal animation. Good luck to anyone who attempts an SDL2 Spine extension, and hopefully this info is at least a little useful at getting started, and thanks to everyone for the convo and info.
UPDATE: I quickly looked through the source to sdl_gfx to see what his textured polygon function does, and weirdly enough it actually converts the surface to a texture, although I think I understand why he does it that way. He can get raw pixel data from the surface, and perhaps not from the texture.
Another method you could take is just using SDL to render directly to OpenGL primitives, but again that's kind of not a direction I want to take. As much as is possible I like to try to use these engines sort of "as is" without hacking them up too much. Perhaps, for me, it's reason enough to consider changing directions altogether again back to libgdx, who knows.
