It's possible with a few limitations. Unity 4.6 with the new UI system just released a few days ago.
Note: Spine-Unity uses a GameObject with a MeshRenderer to render Spine skeletons.
Few things that seem to be true:
1) You can relative-position GameObjects with MeshRenderers under UI Canvases by parenting your GameObject to another Rect Transform, or by changing your Spine GameObject's regular transform itself into a Rect Transform.
2) The Screen Space - Overlay
Canvas Render Mode doesn't seem to render meshes. But you still have the option of using Screen Space - Camera
and World Space
I haven't tried it myself but it looks like it'll be usable.
Someone should look into how draw order/sorting order is determined between regular UI elements and MeshRenderers in this case.