You would have to choose the right silicone as most (all?) contain nasties that are given off as smelly fumes as the silicone cures and those fumes will be trapped inside the box. The vinegar smell (acetoxy cure) produces an acetic acid byproduct and that is corrosive to brass and copper. Better to use gaskets made from silicone as they are already cured and permit the box to be opened.
Are the boxes themselves 100% airtight or will gas molecules diffuse through microscopic pores?
If you really wanted to store the boxes then purge them with inert gas and place in bottling jars,purge the jar and screw the lid on. If they can store peaches for years without oxidation then they will store coins as well.