Any coat oversprayed on a metallic finish will change the reflective properties of the paint. You can spray Future and then a semi-gloss or matte coat over the decals while they're still on the sheet. You then have to trim them carefully, without cutting through the paper, as the overcoat on the sheet will give you a layer across the entire sheet.
Another alternative would be brushing on Future, then your final finish to the decals only, after they have been applied. (the Future will protect the decals from a lacquer dullcoat) The stencils could be tricky, but the larger decals would be easier.
If you try this, try it on scrap first, to see if it gives you the resuts you desire.
Good luck !