Does the metal need to be real rusty? I used a gallon of white vinegar, and put in a small handful of nails that had some rust on them, let it sit and stew for about 4 days, couldn't see any difference in the vinegar, and all it did was get the leather damp.
I just tossed in a chunk of 000 steel wool that I have, and going to let it sit for a few more days, but didn't know if the rust was a key component??