Justin Ouellette took two washing soda-based DIY film developers out for a test: coffee (aka caffeinol or folgernol) and vitamin C, with Rodinal as a reference.
I tried coffee once and got no development whatsoever. While this seems uncommon as I’ve never heard or read about this happening to anyone else, it does illustrate the [hopefully obvious] point that testing weird developers on rolls that you actually care about is a bad idea. Against that hope of obviousness, I tested it on a roll I was really excited about and learned the lesson the hard way. Dumbass. I haven’t tried it again, but Justin’s results are inspiring enough to make me want to give it another go.