@Vambrite This oversight has just been fixed, Custom Slot Materials can now be set via SkeletonGraphicCustomMaterials.
For SkeletonGraphic, please note that CustomSlotMaterials will use the texture of the assigned replacement material, not the actual material (that's due to how MeshGenerator and CanvasRenderer work).
A new spine-unity 4.3 unitpackage is available:
https://esotericsoftware.com/spine-unity-download