Gadget, I too have an underexposed 120 neg that can't be taken again. It was developed in Pyrocat-HD, so it already has some stain. It is of normal or slightly more contrast. (there was nothing wrong with the developer, the film was definitely underexposed)
Have you any suggestions? I was thinking of acquiring the ingredients for VMI mercury intensifier. I know how to dispose of the used solution, by the way.