You have ruled out a lot of things, which makes escaping pretty hard. The only ways I've seen someone escape from prison are; a weakness in the structure, outside help, magic or attacking guard walking past. Oh and of course getting a dog to help, ala monkey island
I would find it totally unrealistic to make a lockpick from grinding stones. Has this been done in a film/another game?
However, what about if you could find some rat bones under your bed that could be used for lock picking. Move bed, search area, find bones, pick correct combination of bones to pick lock.
From what I've read I also don't think you have mentioned how the pow gets fed and where he, er, does his business if you know what I mean. This info could maybe help in coming up with more suggestions.