You could first develop the film to complete black and then use a copper-sulfate & hydrogen peroxide bleach. This will dissolve all the gelatin in contact with developed silver. This would be pretty damn easy methinks, as long as you have some kind of developer. In fact, if you used "Caffenol" or similar, you could probably get all of your materials at a grocery/pharmacy.
edit: It just occurred to me that this would only work for unprocessed plates. If you're using a pre-existing color image, then the bleach would in fact get rid of the silver areas, but it would presumably leave the silver-free areas with gelatin and the thickness would vary then.
Knowing what the varnish is made out of seems an important step. What about Shaz's thread, or Bertrand's book; do either say?