Hmm... Camera and Darkroom .. once had a *very informative article about using extremely "de-rated" film speeds to expand tonal range ... I think it was entitled "Taming Extreme Contrast" - or something like that. If memory serves -- essentailly, the film was overexposed by six or seven stops and processed *very* dilute developer.
I don't remember what film was being used - I do remember effective film speeds of ISO 3 - 6.
When I get a chance, I'll try to look it up.
You didn't say what film format was used. Is it possible to cut a piece off and try it?