I decided to test Caffenol-c tonight...
500 ml of water (for 120)
1 gram Vitamin C
4 teaspponns washing soda
8 teaspoons instant coffee

I tested it on J&C Pro 100 film, exposed at approx 100 (I giuessed most exposures) for 25 minutes

The negatives look pretty good, the stain is there but les pronounced than my previous test that required longer times (30-45 minutes)
I think that some 10-15 extra minutes would increase the stain and improve the shadow density

I'll try to scan the negatives in a flat bed and let you know what happens