1) Seal the entire dag in casting resin
2) If having the resin touch the developed surface destroys the dag, put a glass cover over the image and seal it well enough to keep out the resin and then seal that whole package in resin. It sure wouldn’t look antique.
3) Seal the dag inside a glass container, melt it shut. (How much heat can they take? Any?
4) Use helium or nitrogen to displace the air before sealing as a way to drive out oxygen and sulfer compounds. Less reactive atmosphere means less corrosion.
5) Put the dag in a jar and immerse it in a non-reactive fluid – optical grade mineral oil maybe – the play of light through the fluid changes the experience of viewing the image.
6) Instead of vacuum sealing them – vacuums are notoriously hard to maintain, seal them while they are in a super-dehumidified chamber – not sure how to achieve that simply however.