custom vertex shader that works with skinned mesh

The two Mainquestions:

1. Can I - in the vertex shader - access the ORIGINAL, UNTRANSFORMED vertex position of a skinned mesh? (additionally to the already skinned vertex position, which is streamed in vertex : POSITION)

2. How can I access the blendweights of a vertex in the vertexshader, and are these also accessible if the game object has a normal mesh renderer applied instead of a skinned mesh renderer?

background:

I need to write a custom shader that works with a skinned mesh on mobile platforms. It wouldn't be actually a very complex shader, just calculating some texture coorinates in the vertex shader, and displaying a texture in the fragment shader.

The problem: The calculated texture coordinates depend upon the original vertex position BEFORE the skinning transformation is applied. They are also not constant for each vertex, as they also depend on camera position (in particular, the texture coordinates represent a flat camera projection onto the untransformed mesh).

I read (https://docs.unity3d.com/Documentation/Components/class-SkinnedMeshRenderer.html) that on mobile devices the Skinning transformation happens on the cpu or on the floatingpoint coprocessor respectively - so I assume that the position sream that the vertexshader receives already contains the transformed vertex coordinates instead of the original positions.

So - how can I achieve what I need? How can I access in the vertex shader the original vertex position?

If this is not possible I will try to do the skinning transformation myself in the vertex shader - in this case, how can I access the skinning weights of the vertices in the vertexshader and circumvent the transformation on the coprocessor?

thank you for the help!