I've never tried to put it over a decal John, so I can't give you an answer based on experience, but I would have thought that it probably wouldn't work. The thickness of the melamine finish is probably less than the thickness of the decal at a guess, so you're going to end up with a ridge I would think.
As to what you stick on the decal with - no idea. There are those with much more experience and expertise in that area than me, so hopefully one of them will pop along soon to shed some light.