Kestrel,
You have the procedure down pat. Im not aware of any other technique that would bring about the same result, the reason being that a flat surface usually results in air being trapped beneath the decal which would then give you the silvering effect. Another problem is that decal films are usually glossy, so the decals would stand out on the flat surface without a flat clear coat.
You may want to try applying decals on flat surfaces using thinned Elmer's Glue (thin with water at about 70% water and 30% glue) as a medium. That should limit the silvering but not the glossy decal surface. You could then spray on flat clear to even out the finish.
Hope this helps.