Aghhh thanks for the positive feedback. I think you'd have to have the effects use a different spine skeleton that uses the spine premultiplied-alpha shader and just layer them on top of the character.
Or depending upon how complex your effects are you could just import and animate the sprites in unity itself and give them a material with the particles-additive shader. I don't think there's anyway to get additive lighting working using the alpha cutout material using the one spine skeleton.
I'll try put together some notes on it if I get some free time.
I've also realised that using spine flip doesn't work for the shader I suggested using above.To get it to render correctly you have to use a double sided material which renders normals on both sides of a mesh. The toony colours shader has a shader generator tool which I used to test this and it worked but it wasn't the exact same shader and I lost some of the soft lighting.
Todd River's sprite shader's can render both sides of a mesh too which is really cool. They are a really feature complete shader set. Sprite Shaders for Unity
The other solution is to scale in x to flip the character but I don't know if that would break other things. I'm going to do some further research and will append my notes with the solutions I've found on this at some point too.
