I only have experience with exposed plates, but I think water (or some other liquid) would remove it off from the plate. I'd prefer just to cut a few new glass sheets in the same size (or have someone to do it).
If they have been kept in the dark and are not exposed, they may be of interest to someone here on APUG. There are a few people around who indulge in exposing old glass plates.
EDIT: btw, if you decide to cut plates yourself, be aware that the edges are very sharp. Use something to sand them down a bit so you can handle them safely. Glass cuts can be horrible.