Haha, that one seems like it might get complicated. You'd probably want to build a MonoBehaviour that lives on a game object which also has a rect transform and a skeletal animation, and have the MonoBehaviour transform the skeletal animation to the rect transform's bounds every update. I have no idea what hitches would be run onto on the way though, haven't tried this myself. Sorry 🙁 You'd also probably want to write a custom spine shader based on the UI shaders
when I checked it seemed like a lot of extra data was being passed to the UI shaders through Unity's shader property system (hopefully I'm remembering what it's called correctly), stuff which you might want the skeletal animation to be affected by. This might be where ui draw order is handled? I actually have no idea.
Doing it sort of on top of the current system seems better than doing something like rendering the spine animation to a render target and then using that texture as a raw image uGUI component, which would probably have much more limited uses.
Sorry I can't be more help. If I end up wanting this myself, I'll post how my attempt goes.
