I found a way to integrate the feature into AnimationState, now in git. It works like this:
state.setAnimation(0, "walk", true);
TrackEntry entry = state.setAnimation(1, "jump", true);
entry.mix = 0.4f;
Or:
state.setAnimation(0, "walk", true);
state.setAnimation(1, "jump", true).mix = 0.4f;
Each frame, walk will be applied on track 0, then jump will be applied on track 1 with a mix of 0.4, meaning it will use 60% of whatever pose is the result of the previous tracks (here will be the walk pose) and 40% of the jump animation. If mix isn't set 1 is used, meaning overwrite any previous tracks. As before, only keyed properties are affected by an animation.
Note you can keep a reference to the TrackEntry and change the mix over time as you wish to get various mixing effects. Though with spine-c be careful not to use the TrackEntry after it is disposed by AnimationState.