Just posting to say that I'm trying to get this working myself! I have an entirely script-based Unity Framework that I've built up over the years (I don't like the way scenes and prefabs work as they are extremely hard to merge if you're working on a team, and they are generally impossible to port to other frameworks which I often have to do when I'm done prototyping in Unity).
That said! I'm kinda close. I'm getting the same error you are at this point and am working on a clean patch to the spine-unity classes to get it cleaned up. Should have something tonight!
Also, as for why I'd want to do things this way, I have a ton of characters in this game with minor randomized variations. To do that I need to switch what textures and anims they point to dynamically, and ideally I'd have everything working entirely in script so I can set breakpoints and actually see what's going wrong with dudes. Anyway!
