I once experimented with contacting a B&W print onto another piece of RC paper and using that as the negative for a cyanotype. The exposure was very long, like half an hour and the print was still rather light. I was using sunlight in those days. You logic is sound as a direct positive of a negative will yield a negative. I'm not sure how modern gelatin silver papers would take to waxing or oiling. It's hard to imagine waxing RC paper. I'd go with ortho film. It's an extra step, but the frustration factor will be lower.