I have now added an additional example scene
SpriteMask and RectMask2D to the spine-unity unitypackage under
Spine Examples/Other Examples.
You can download it here as usual:
Spine Unity Download
Please let us know if it helped you with resolving your problem.
In general: please note that RectMask2D is for UI elements (SkeletonGraphic) and SpriteMask for non-UI elements (SkeletonAnimation and SkeletonMecanim). This is general behaviour in Unity, treating UI elements differently from non-UI elements such as Sprites.