Ok - I think I got it:
In order to get rid of dummy node without losing skinning info:
my suggestion would be to dupicate and copy the skin weights over.
- duplicate your rig.
- freeze the transforms you need to freeze.
- duplicate the mesh.
- bind your duplicate mesh to the duplicate rig.
- select the original mesh and shift+select the duplicate mesh
- skin > edit smooth skin > copy skin wieghts [option box]
- in the copy skin weights window choose edit > reset
- click copy