Some of them definitely are available. Actor/Object/Pivot Position and Orientation seem to be. But based on your other post, if the decal is on a moving actor those may not be suitable. Bounds and Radius did not work at all, nor did any scale based method I tried.
But it can sample some scene textures, notably custom stencil, which could possibly be exploited for a small cost. This would only allow one value per decal receiver at a time, but could easily allow an array to select a different index depending upon what object it was put onto.
It also can sample world normals, which is useful for changing the behavior based upon the surface the decal is applied to (ie different atlas index for ground vs wall) but also not particularly useful for most moving objects.
You could also use a static mesh with the node “static mesh decal function” instead of a decal. This allows a mesh to project a texture to a surface in a manner similar to a decal. Then you would have access to Prim Data or instanced custom data for instanced meshes. Probably more expensive and less convenient than a decal, so I’m not sure if it’s worth it unless the draw calls are a big problem for you.