I made converter that outputs native QML items and animations out of Spine exported json files. So in a way, this is not a runtime at all, but the end results are similar, but with an added benefit of having native QML code. This means you could make a skeleton animate inside another skeleton or make some parts of the animation be procedural. Or you could just like the native QML look.
Here is an example of it running: http://www.youtube.com/watch?v=jizHTY3FRvI
And here is the source code: https://github.com/vmatikainen/spine-qml-converter
It lacks the IK and FFD features, but I guess they could be cannibalized (in a nice way) from QML runtime and integrated as native QML items.
Feel free to share your thoughts and compliments 8)